Don't trigger -Wuser-defined-literals for system headers

Summary:
In D41064, I proposed adding `#pragma clang diagnostic ignored
"-Wuser-defined-literals"` to some of libc++'s headers, since these
warnings are now triggered by clang's new `-std=gnu++14` default:

```
$ cat test.cpp
#include <string>

$ clang -std=c++14 -Wsystem-headers -Wall -Wextra -c test.cpp
In file included from test.cpp:1:
In file included from /usr/include/c++/v1/string:470:
/usr/include/c++/v1/string_view:763:29: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
    basic_string_view<char> operator "" sv(const char *__str, size_t __len)
                            ^
/usr/include/c++/v1/string_view:769:32: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
    basic_string_view<wchar_t> operator "" sv(const wchar_t *__str, size_t __len)
                               ^
/usr/include/c++/v1/string_view:775:33: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
    basic_string_view<char16_t> operator "" sv(const char16_t *__str, size_t __len)
                                ^
/usr/include/c++/v1/string_view:781:33: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
    basic_string_view<char32_t> operator "" sv(const char32_t *__str, size_t __len)
                                ^
In file included from test.cpp:1:
/usr/include/c++/v1/string:4012:24: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
    basic_string<char> operator "" s( const char *__str, size_t __len )
                       ^
/usr/include/c++/v1/string:4018:27: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
    basic_string<wchar_t> operator "" s( const wchar_t *__str, size_t __len )
                          ^
/usr/include/c++/v1/string:4024:28: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
    basic_string<char16_t> operator "" s( const char16_t *__str, size_t __len )
                           ^
/usr/include/c++/v1/string:4030:28: warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]
    basic_string<char32_t> operator "" s( const char32_t *__str, size_t __len )
                           ^
8 warnings generated.
```

Both @aaron.ballman and @mclow.lists felt that adding this workaround to
the libc++ headers was the wrong way, and it should be fixed in clang
instead.

Here is a proposal to do just that.  I verified that this suppresses the
warning, even when -Wsystem-headers is used, and that the warning is
still emitted for a declaration outside of system headers.

Reviewers: aaron.ballman, mclow.lists, rsmith

Reviewed By: aaron.ballman

Subscribers: mclow.lists, aaron.ballman, andrew, emaste, cfe-commits

Differential Revision: https://reviews.llvm.org/D41080

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320755 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 7 insertions, 0 deletions

diff --git a/test/SemaCXX/no-warn-user-defined-literals-in-system-headers.cpp b/test/SemaCXX/no-warn-user-defined-literals-in-system-headers.cpp
new file mode 100644
index 0000000000..ee5b0c47b9
--- /dev/null
+++ b/test/SemaCXX/no-warn-user-defined-literals-in-system-headers.cpp
@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -Wsystem-headers -isystem %S %s
+
+#include <no-warn-user-defined-literals-in-system-headers.h>
+
+void operator "" bar(long double); // expected-warning{{user-defined literal suffixes not starting with '_' are reserved}}
diff --git a/test/SemaCXX/no-warn-user-defined-literals-in-system-headers.h b/test/SemaCXX/no-warn-user-defined-literals-in-system-headers.h
new file mode 100644
index 0000000000..d1e2583168
--- /dev/null
+++ b/test/SemaCXX/no-warn-user-defined-literals-in-system-headers.h
@@ -0,0 +1,2 @@
+// Header for no-warn-user-defined-literals-in-system-headers.cpp
+void operator "" foo (const char *);