Use the archive offset with --whole-archive.

The test ELF/lto/thin-archivecollision.ll was not testing what it
wanted to test. It needs two archive members with the same name, but
different offsets.

Without this we could remove all references of OffsetInArchive and all
tests would still pass.

Fixing the test showed that the --whole-archive case was broken, which
this patch fixes.

llvm-svn: 302241