[llvm-readobj] - Refactor ELFDumper<ELFT>::getStaticSymbolName.

This is a followup for D83129.
It is possible to make `getStaticSymbolName` report warnings inside
and return the "<?>" on a error. This allows to encapsulate errors handling
and slightly simplifies the logic in callers code.

Differential revision: https://reviews.llvm.org/D83208