Use simple section names for COMDAT sections on COFF.

With this patch we use simple names for COMDAT sections (like .text or .bss).
This matches the MSVC behavior.

When merging it is the COMDAT symbol that is used to decide if two sections
should be merged, so there is no point in building a fancy name.

This survived a bootstrap on mingw32.

llvm-svn: 195798