[lld-macho] Extend ICF to literal sections

Literal sections can be deduplicated before running ICF. That makes it
easy to compare them during ICF: we can tell if two literals are
constant-equal by comparing their offsets in their OutputSection.

LLD-ELF takes a similar approach.

Reviewed By: #lld-macho, gkm

Differential Revision: https://reviews.llvm.org/D104671