[lld-macho] Avoid force-loading the same archive twice

We need to dedup archive loads (similar to what we do for dylib
loads).

I noticed this issue after building some Swift stuff that used
`-force_load_swift_libs`, as it caused some Swift archives to be loaded
many times.

Reviewed By: #lld-macho, thakis, MaskRay

Differential Revision: https://reviews.llvm.org/D104353