[PGO] Don't group COMDAT variables for compiler generated profile variables in ELF

With this patch, compiler generated profile variables will have its own COMDAT
name for ELF format, which syncs the behavior with COFF. Tested with clang
PGO bootstrap. This shows a modest reduction in object sizes in ELF format.

Differential Revision: https://reviews.llvm.org/D68041

llvm-svn: 373241