[COFF] Fix to not add archive name to buffer identifiers when they come

from thin archives.

Currently lld adds the archive name to MemoryBufferRef identifiers in order to
ensure they are unique. For thin archives, since the file name is already unique and we
want to keep the original path to the file, don't add the archive name.

Differential Revision: https://reviews.llvm.org/D67295

llvm-svn: 371778