CodeGen: Assign an appropriate comdat to thunks.

Previously we were not assigning a comdat to thunks in the Microsoft ABI,
which would have required us to emit these functions outside of a comdat.
(Due to an inconsistency in how we were emitting objects, we were getting this
right most of the time, but only when compiling with function sections.) This
code generator change causes us to create a comdat for each thunk.

Differential Revision: http://reviews.llvm.org/D10829

llvm-svn: 241102