Move demangling function from llvm-objdump to Demangle library

This allows it to be used in an upcoming llvm-readobj change.

A small change in internal behaviour of the function is to always call
the microsoftDemangle function if the string does not have an itanium
encoding prefix, rather than only if it starts with '?'. This is
harmless because the microsoftDemangle function does the same check
already.

Reviewed by: grimar, erik.pilkington

Differential Revision: https://reviews.llvm.org/D56721

llvm-svn: 351448