Support multiple COFF sections with the same name but different COMDAT.

This is the first step to fix pr17918.

It extends the .section directive a bit, inspired by what the ELF one looks
like. The problem with using linkonce is that given

.section foo
.linkonce....

.section foo
.linkonce

we would already have switched sections when getting to .linkonce. The cleanest
solution seems to be to add the comdat information in the .section itself.

llvm-svn: 195148