[lld-macho] Ensure cached objects are affected by `-object_path_lto`

This refactors LTO compile to look more like COFF, where cache hits and misses are all funneled through the same code path.

Previously, cache hits were *not* being saved to -object_path_lto, which led to them sometimes falling out of the cache before dsymutil could process them. As a side effect of the refactor, cached objects are now saved with -save-temps as well, which seems desirable.

(Deleted lld/test/MachO/lto-cache-dsymutil.ll and rolled it into lld/test/MachO/lto-object-path.ll, since the cache-only, non object path approach is unreliable anyway).

Differential Revision: https://reviews.llvm.org/D131624