Object: Improve COFF irsymtab comdat representation.

Change the representation of COFF comdats so that a COFF linker
is able to accurately resolve comdats between IR and native object
files. Specifically, apply name mangling to comdat names consistently
with native object files, and do not export comdats with an internal
leader because they do not affect symbol resolution.

Differential Revision: https://reviews.llvm.org/D40278

llvm-svn: 318805