[llvm-readelf/obj] - Use `RelSymbol<ELFT>` instead of std::pair. NFCI.

We have the `RelSymbol<ELFT>` struct and can use it instead
of `std::pair<const Elf_Sym *, std::string>` in a few methods.
This is a bit cleaner.

Differential revision: https://reviews.llvm.org/D87092