[clang] Support __attribute__((ifunc(...))) on Darwin platforms

Unlike ELF targets, MachO does not support the same kind of dynamic symbol
resolution at load time.  Instead, the corresponding MachO feature resolves
symbols lazily on first call.

Reviewers:
JDevlieghere, dmpolukhin, ahmedbougacha, tahonermann, echristo, MaskRay, erichkeane

Reviewed By: MaskRay, echristo, ahmedbougacha

Pull Request: https://github.com/llvm/llvm-project/pull/73687