CodeGen: Emit some functions as weak_odr under -fms-compatibility

Summary:
MSVC always emits inline functions marked with the extern storage class
specifier.  The result is something similar to the opposite of
__attribute__((gnu_inline)).

This extension is also available in C.

This fixes PR19264.

Reviewers: rnk, rsmith

CC: cfe-commits

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