[llvm-readobj][ELF]Add demangling support

This change adds demangling support to the ELF side of llvm-readobj,
under the switch --demangle/-C.

The following places are demangled: symbol table dumps (static and
dynamic), relocation dumps (static and dynamic), addrsig dumps, call
graph profile dumps, and group section signature symbols.

Although GNU readelf doesn't support demangling, it is still a useful
feature to have, and brings it on a par with llvm-objdump's
capabilities.

This fixes https://bugs.llvm.org/show_bug.cgi?id=40054.

Reviewed by: grimar, rupprecht

Differential Revision: https://reviews.llvm.org/D56791

llvm-svn: 351450