[lld-macho] Simplify archive loading logic

This is a follow-on to {D129556}. I've refactored the code such that
`addFile()` no longer needs to take an extra parameter. Additionally,
the "do we force-load or not" policy logic is now fully contained within
addFile, instead of being split between `addFile` and
`parseLCLinkerOptions`. This also allows us to move the `ForceLoad` (now
`LoadType`) enum out of the header file.

Additionally, we can now correctly report loads induced by
`LC_LINKER_OPTION` in our `-why_load` output.

I've also added another test to check that CLI library non-force-loads
take precedence over `LC_LINKER_OPTION` + `-force_load_swift_libs`. (The
existing logic is correct, just untested.)

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D130137