Put profile variables of COMDAT functions to it's own COMDAT group.

In -fprofile-instr-generate compilation, to remove the redundant profile
variables for the COMDAT functions, these variables are placed in the same
COMDAT group as its associated function. This way when the COMDAT function
is not picked by the linker, those profile variables will also not be
output in the final binary. This may cause warning when mix link objects
built w and wo -fprofile-instr-generate.

This patch puts the profile variables for COMDAT functions to its own COMDAT
group to avoid the problem.

Patch by xur.
Differential Revision: http://reviews.llvm.org/D12248

llvm-svn: 248440