[llvm-objdump] Change errors to warnings for symbol section name dumping

Port D69671 (llvm-readobj) to llvm-objdump. Add a class llvm::objdump::Dumper
and move some free functions into Dumper so that they can call
reportUniqueWarning.

Warnings seems preferable in these cases as the issue is localized and we can
continue dumping other information.

Differential Revision: https://reviews.llvm.org/D154754