Use proper section suffix for COFF weak symbols

32-bit symbols have "_" as global prefix, but when forming the name of
COMDAT sections this prefix is ignored. The current behavior assumes that
this prefix is always present which is not the case for 64-bit and names
are truncated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187356 91177308-0d34-0410-b5e6-96231b3b80d8