[lld-macho] Set FinalDefinitionInLinkageUnit on most LTO externs

Since Mach-O has a two-level namespace (unlike ELF), we can usually set
this property to true.

(I believe this setting is only available in the new LTO backend, so I
can't really use ld64 / libLTO's behavior as a reference here... I'm
just doing what I think is correct.)

See {D119294} for the work done to calculate the `interposable` used in
this diff.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D119506