[lld-macho] Tweak the names we give to archive members

In particular, make it `foo.a(foo.o)$ARCHIVE_OFFSET`. The goal is to
make it more similar to both ld64 implementation, which uses the
`foo.a(foo.o)$MODULE_ID` format. We dump some of these names in LTO
code, so matching ld64's format is helpful. This format is also more
similar to LLD-ELF's, which is `foo.a(foo.o at $ARCHIVE_OFFSET)`.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D148828