[llvm-objdump] - Demangle the symbols when printing symbol table and relocations.

This is https://bugs.llvm.org/show_bug.cgi?id=40009,

llvm-objdump does not demangle the symbols when prints symbol
table and/or relocations.

Patch teaches it to do that.

Differential revision: https://reviews.llvm.org/D55821

llvm-svn: 349613