[COFF] Cope with GCC produced weak aliases referring to comdat functions

For certain cases of inline functions written to comdat sections,
GCC 5.x produces a weak symbol in addition, which would end up
undefined in some cases.

This no longer seems to happen with GCC 6.x or newer though.

Differential Revision: https://reviews.llvm.org/D52602

llvm-svn: 343877