This change adds demangling support to the ELF side of llvm-readobj, under the switch --demangle/-C. It relies on a refactor proposed in D56721.
The following places are demangled: symbol table dumps (static and dynamic), relocation dumps (static and dynamic), addrsig dumps, call graph profile dumps, and group sections.
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.