[llvm-readobj] - Do not fail to dump the object which has wrong type of .shstrtab.

Imagine we have object that has .shstrtab with type != SHT_STRTAB.
In this case, we fail to dump the object, though GNU readelf dumps it without
any issues and warnings.

This patch fixes that. It adds a code to ELFDumper.cpp which is based on the implementation of getSectionName from the ELF.h:

https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Object/ELF.h#L608
https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Object/ELF.h#L431
https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Object/ELF.h#L539

The difference is that all non critical errors are ommitted what allows us to
improve the dumping on a tool side. Also, this opens a road for a follow-up that
should allow us to dump the section headers, but drop the section names in case if .shstrtab is completely absent and/or broken.

Differential revision: https://reviews.llvm.org/D63266

llvm-svn: 363371