[PATCH] CodeGen: Emit some functions as weak_odr under -fms-extensions

[Reid Kleckner]

  > @rnk: If mingw people are using `-fms-extensions`, they will not be effected at all in the C language mode because we require linkage to be `GVA_CXXInline`; those people will continue to get either their C99 or GNU semantics.  They would only be effected if the used C++ or `__inline`.  AFAICT, this extension is conforming if surprising.

  Sounds good.  Note that we actually use GVA_CXXInline for inline functions in C mode when -fms-compatibility is enabled, but then we're already in quirks mode so it's OK.

http://llvm-reviews.chandlerc.com/D3207