[llvm-readobj/llvm-readelf] - .stack_sizes: demangle symbol names in warnings reported.

I started this patch as a refactoring, tried to make a helper for
getting symbol names, similar to how we get section names
used in warning messages.

So this patch cleanups the code and fixes an issue: symbol names
in warning messages were not demangled.

Differential revision: https://reviews.llvm.org/D68012

llvm-svn: 372867