[lld-macho] Have ICF dedup explicitly-defined selrefs

This is what ld64 does (though it doesn't use ICF to do this; instead it
always dedups selrefs by default).

We'll want to dedup implicitly-defined selrefs as well, but I will leave
that for future work.

Additionally, I'm not *super* happy with the current LLD implementation
because I think it is rather janky and inefficient. But at least it
moves us toward the goal of closing the size gap with ld64. I've
described ideas for cleaning up our implementation here:
https://github.com/llvm/llvm-project/issues/57714

Differential Revision: https://reviews.llvm.org/D133780