[lld-macho] In the context of relocs, s/target/referent/ for sections & symbols

The word "target" is overloaded, so lighten its load by using another word to denote the symbol or section to which a reloc points. While more stilted than "target", "referent" is rather less pompous than "designatum" or "denotatum". :P

Along the way, make a few neighboring variable names more descriptive.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D87584