ExceptionDemo: prefer headers over redeclarations

Rather than redeclaring the interfaces for exceptions, prefer using the
`unwind.h` header.  This is vended by at least gcc and clang, and can also be
found by an external unwinding library (e.g. libunwind).  Doing this simplifies
the example to the exception handling itself.  Minor tweaks are the result of
_Unwind_Context_t not being defined, which is just a typedef for struct
_Unwind_Context *.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287478 91177308-0d34-0410-b5e6-96231b3b80d8

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