[lld/mac] Implement removal of unused dylibs

This omits load commands for unreferenced dylibs if:
- the dylib was loaded implicitly,
- it is marked MH_DEAD_STRIPPABLE_DYLIB
- or -dead_strip_dylibs is passed

This matches ld64.

Currently, the "is dylib referenced" state is computed before dead code
stripping and is not updated after dead code stripping. This too matches ld64.
We should do better here.

With this, clang-format linked with lld (like with ld64) no longer has
libobjc.A.dylib in `otool -L` output. (It was implicitly loaded as a reexport
of CoreFoundation.framework, but it's not needed.)

Differential Revision: https://reviews.llvm.org/D103430