[lld-macho] Simplify linker optimization hint processing

This commit removes the `relocTargets` vector, and instead makes the
code reconstruct the referent addresses from the relocated instructions.
This will allow us to move `applyOptimizationHints` from
`ConcatInputSection::writeTo` to a separate pass that parses and applies
LOHs in one step, on a per-file basis. This will improve performance, as
parsing is currently done serially in `ObjFile::parse`.

I opted to remove the sanity check that ensures that all relocations
within a LOH point to the same symbol. This completely eliminates the
need to search through relocations. It is my understanding that
mismatched relocation targets should not be present in valid object
files, so it's unlikely that the removal will lead to mislinks.

Differential Revision: https://reviews.llvm.org/D133274