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

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@205485 91177308-0d34-0410-b5e6-96231b3b80d8