[lld-macho] -weak_{library,framework} should always take priority

We were not setting forceWeakImport for file paths given by
`-weak_library` if we had already loaded the file. This diff fixes that
by having `loadDylib` return a cached DylibFile instance even if we have
already loaded that file.

We still avoid emitting multiple LC_LOAD_DYLIBs, but we achieve this by
making inputFiles a SetVector instead of relying on the `loadedDylibs`
cache.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D93255