Add a missing declaration to our unwind.h implementation. This is

necessary to be fully compatible with existing software that calls into
the linux unwind code. You can find documentation of this API and why it
exists in the discussion abot NPTL here:
https://gcc.gnu.org/ml/gcc-patches/2003-09/msg00154.html

llvm-svn: 224933

clang/lib/Headers/unwind.h

@@ -199,6 +199,8 @@ _Unwind_Word _Unwind_GetIPInfo(struct _Unwind_Context *, int *);
 
 _Unwind_Word _Unwind_GetCFA(struct _Unwind_Context *);
 
+_Unwind_Word _Unwind_GetBSP(struct _Unwind_Context *);
+
 void *_Unwind_GetLanguageSpecificData(struct _Unwind_Context *);
 
 _Unwind_Ptr _Unwind_GetRegionStart(struct _Unwind_Context *);