[llvm-readobj] Add new helper function `getSymbolVersionByIndex()`

Summary: When implementing `GNU style` dumper for `.gnu.version` section, we should find symbol version name by `vs_index`.

Reviewers: jhenderson, rupprecht

Reviewed By: rupprecht

Subscribers: arphaman, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59545

llvm-svn: 357164