[PGO] Fix profile var comdat generation problem with COFF

When targeting COFF, it is required that a comdat section to
have a global obj with the same name as the comdat (except for
comdats with select kind to be associative). This fix makes
sure that the comdat is keyed on the data variable for COFF.

Also improved test coverage for this.

llvm-svn: 256193