[libc++] Add _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY to support GCC ABI compatibility

Summary:
GCC and Clang handle visibility attributes on the out-of-line definition of externally instantiated templates differently. For example in the reproducer below Clang will emit both 'foo' and 'bar' with default visibility while GCC only emits a non-hidden 'foo'.  

```
// RUN: g++ -std=c++11 -shared -O3 test.cpp && sym_extract.py a.out
// RUN: clang++ -std=c++11 -shared -O3 test.cpp && sym_extract.py a.out
#define INLINE_VISIBILITY __attribute__((visibility("hidden"), always_inline))

template <class T>
struct Foo {
  void foo();
  void bar();
};

template <class T>
void Foo<T>::foo() {}

template <class T>
inline INLINE_VISIBILITY
void Foo<T>::bar() {}

template struct Foo<int>;
```

This difference creates ABI incompatibilities between Clang and GCC built dylibs. Specifically GCC built dylibs lack definitions for various member functions of `basic_string`, `basic_istream`, `basic_ostream`, `basic_iostream`, and `basic_streambuf` (All of these types are externally instantiated). 

Surprisingly these missing symbols don't cause many problems because the functions are marked `always_inline`  therefore the dylib definition is rarely needed. However when an out-of-line definition is required then GCC built dylibs will fail to link. For example [GCC built dylibs cannot build Clang](http://stackoverflow.com/questions/39454262/clang-build-errors).

This patch works around this issue by adding `_LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY` which is used to mark externally instantiated member functions as always inline. When building the library `_LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY` sets the symbol's visibility to "default" instead of "hidden", otherwise it acts exactly the same as `_LIBCPP_INLINE_VISIBILITY`.

After applying this patch GCC dylibs now contain:
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE7sungetcEv`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE5gbumpEi`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE7sungetcEv`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE9sputbackcEc`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEE3getERNS_15basic_streambufIwS2_EE`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEElsEPFRNS_9basic_iosIwS2_EES6_E`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE4setpEPcS4_`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEEC1EPNS_15basic_streambufIwS2_EE`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE6snextcEv`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEE4swapERS3_`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEE4swapERS3_`
  * `_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE6__initEPKcm`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEErsEPFRNS_8ios_baseES5_E`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE9pubsetbufEPcl`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE10pubseekoffExNS_8ios_base7seekdirEj`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEErsEPFRNS_9basic_iosIwS2_EES6_E`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE5pbumpEi`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEE5seekpENS_4fposI11__mbstate_tEE`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEE7getlineEPcl`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE5sgetcEv`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEE3getERNS_15basic_streambufIcS2_EE`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEElsEPFRNS_8ios_baseES5_E`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE8in_availEv`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEErsEPFRNS_8ios_baseES5_E`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE6sbumpcEv`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEElsEPFRNS_9basic_iosIcS2_EES6_E`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEE3getERc`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE6snextcEv`
  * `_ZNSt3__112basic_stringIwNS_11char_traitsIwEENS_9allocatorIwEEE6__initEmw`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEE7getlineEPwl`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEE5tellpEv`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEE3getERw`
  * `_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE6__initEmc`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE7pubsyncEv`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEE3getEPcl`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEEC2EPNS_15basic_streambufIcS2_EE`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEErsEPFRNS_9basic_iosIcS2_EES6_E`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE7pubsyncEv`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE5sputcEc`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEE5seekpExNS_8ios_base7seekdirE`
  * `_ZNKSt3__115basic_streambufIcNS_11char_traitsIcEEE6getlocEv`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE5gbumpEi`
  * `_ZNSt3__114basic_iostreamIcNS_11char_traitsIcEEE4swapERS3_`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEE5seekpENS_4fposI11__mbstate_tEE`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEE5tellpEv`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEElsEPFRS3_S4_E`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEE3getEPwl`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEEC2EPNS_15basic_streambufIwS2_EE`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEElsEPFRS3_S4_E`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE4setgEPcS4_S4_`
  * `_ZNSt3__112basic_stringIwNS_11char_traitsIwEENS_9allocatorIwEEE6__initEPKwmm`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE4setgEPwS4_S4_`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEEC1EPNS_15basic_streambufIwS2_EE`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE8pubimbueERKNS_6localeE`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEE4swapERS3_`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEEC2EPNS_15basic_streambufIwS2_EE`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE10pubseekposENS_4fposI11__mbstate_tEEj`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE5pbumpEi`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE5sgetcEv`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEE4swapERS3_`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE10pubseekposENS_4fposI11__mbstate_tEEj`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE5sputnEPKcl`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEE5seekpExNS_8ios_base7seekdirE`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE5sgetnEPwl`
  * `_ZNSt3__113basic_ostreamIwNS_11char_traitsIwEEElsEPFRNS_8ios_baseES5_E`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE4setpEPwS4_`
  * `_ZNSt3__115basic_streambufIcNS_11char_traitsIcEEE5sgetnEPcl`
  * `_ZNKSt3__115basic_streambufIwNS_11char_traitsIwEEE6getlocEv`
  * `_ZNSt3__114basic_iostreamIcNS_11char_traitsIcEEEC2EPNS_15basic_streambufIcS2_EE`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE8pubimbueERKNS_6localeE`
  * `_ZNSt3__114basic_iostreamIcNS_11char_traitsIcEEEC1EPNS_15basic_streambufIcS2_EE`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE8in_availEv`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEEC1EPNS_15basic_streambufIcS2_EE`
  * `_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE6__initEPKcmm`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE6sbumpcEv`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE10pubseekoffExNS_8ios_base7seekdirEj`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEEC2EPNS_15basic_streambufIcS2_EE`
  * `_ZNSt3__113basic_istreamIwNS_11char_traitsIwEEErsEPFRS3_S4_E`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE9sputbackcEw`
  * `_ZNSt3__112basic_stringIwNS_11char_traitsIwEENS_9allocatorIwEEE6__initEPKwm`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE5sputnEPKwl`
  * `_ZNSt3__113basic_istreamIcNS_11char_traitsIcEEErsEPFRS3_S4_E`
  * `_ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEEC1EPNS_15basic_streambufIcS2_EE`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE9pubsetbufEPwl`
  * `_ZNSt3__115basic_streambufIwNS_11char_traitsIwEEE5sputcEw`


This patch has no effect on Clang based builds.



Reviewers: mclow.lists, eugenis, danalbert, jroelofs, EricWF

Subscribers: beanz, cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D24600

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@281681 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 2 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7c0f0610e..a80349a11 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -330,6 +330,8 @@ endif()
 # headers
 add_compile_flags_if_supported(-nostdinc++)
 
+add_definitions(-D_LIBCPP_BUILDING_LIBRARY)
+
 # Warning flags ===============================================================
 add_definitions(-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 add_compile_flags_if_supported(