lld-link: Allow backward references between associated comdats

References between associated comdats are invalid per COFF spec, but the newest
Windows SDK contains obj files that have these references
(https://bugs.chromium.org/p/chromium/issues/detail?id=925943#c13). So add back
support for them and add tests for them. The old code handled them fine.

This makes lld-link match the behavior of newer link.exe versions as far as I
can tell. (The behavior before this change matched the behavior of older
link.exe versions.)

This mostly reverts r352254.

Differential Revision: https://reviews.llvm.org/D57387

llvm-svn: 352508