[llvm-objdump] Allow --disassemble-functions to take demangled names

The --disassemble-functions switch takes demangled names when
--demangle is specified, otherwise the switch takes mangled names.

https://bugs.llvm.org/show_bug.cgi?id=41908

Reviewers: jhenderson, grimar, MaskRay, rupprecht

Differential Revision: https://reviews.llvm.org/D63524

llvm-svn: 364121