[lld/mac] Crash even less on undefined symbols with --icf=all

Follow-up to https://reviews.llvm.org/D112643. Even after that change, we were
still asserting if two separate functions that are eligible for ICF (same size,
same data, same number of relocs, same reloc types, ...) referred to
Undefineds. This fixes that oversight.

Differential Revision: https://reviews.llvm.org/D114195