[lld-macho] Disambiguate bitcode files with the same name by archive name/offset in archive

Ported from COFF/ELF; test is adapted from
test/COFF/thinlto-archivecollision.ll

LTO expects every bitcode file to have a unique name. If given multiple bitcode
files with the same name, it errors with "Expected at most one ThinLTO module
per bitcode file".

This change incorporates the archive name, to disambiguate members with the
same name in different archives and the offset in archive to disambiguate
members with the same name in the same archive.

Differential Revision: https://reviews.llvm.org/D106179