[Object, llvm-readelf] - Move the API for retrieving symbol versions to ELF.h

`ELFDumper.cpp` implements the functionality that allows to get symbol versions.
It is used for dumping versioned symbols.

This helps to implement https://bugs.llvm.org/show_bug.cgi?id=48670 ("make llvm-nm -D print version names"):
we can move out and reuse the code from `ELFDumper.cpp`.
This is what this patch do: it moves the related functionality to `ELFFile<ELFT>`.

Differential revision: https://reviews.llvm.org/D94771