[llvm-readobj/llvm-readelf] - Reimplement dumping of the SHT_GNU_verdef section.

Currently we have following issues:
1) We have 2 different implementations with a different behaviors for GNU/LLVM styles.
2) Errors are either not handled at all or we call report_fatal_error with not helpfull messages.
3) There is no test coverage even for those errors that are reported.

This patch reimplements parsing of the SHT_GNU_verdef section entries
in a single place, adds a few error messages and test coverage.

Differential revision: https://reviews.llvm.org/D70495