[LLD][ELF] Implement --discard-* for cases when -r or --emit-relocs are used.

When discarding local symbols with --discard-all or --discard-locals,
the ones which are used in relocations should be preserved. LLD used
the simplest approach and just ignored those switches when -r or
--emit-relocs was specified.

The patch implements handling the --discard-* switches for the cases
when relocations are kept by identifying used local symbols and allowing
removing only unused ones. This makes the behavior of LLD compatible
with GNU linkers.

Differential Revision: https://reviews.llvm.org/D77807