[llvm-readobj/elf] - Refine the implementation of "printFunctionStackSize".

This rewrites the logic to get rid of "ELFSymbolRef" API where possible.
This allowed to handle possible errors better, improve warnings reported and add new ones.
Also 'reportWarning' was replaced with 'reportUniqueWarning'

Differential revision: https://reviews.llvm.org/D92545