[lld-macho] Fix use-after-free in loadDylib()

We were taking a reference to a value in `loadedDylibs`, which in turn
called `make<DylibFile>()`, which could then recursively call
`loadDylibs`, which would then potentially resize `loadedDylibs` and
invalidate that reference.

Fixes PR50101.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D101175