[lld-macho][nfc] Add more tests + comments around ICF + unwind info interaction

While working on {D129830}, I realized that our handling of ICF +
eh_frame combined was untested. Additionally I realized that the comment
explaining why we were safely slicing away the functionAddress reloc
from our compact unwind entries was... insufficient and slightly
misleading. I've tried to clarify it.

Reviewed By: #lld-macho, thevinster

Differential Revision: https://reviews.llvm.org/D129894