Merge changes to ItaniumDemangle over to libcxxabi.

ItaniumDemangle had a small NFC refactor to make some of its
code reusable by the newly added Microsoft demangler.  To keep
the libcxxabi demangler as close as possible to the master copy
this refactor is being merged over.

Differential Revision: https://reviews.llvm.org/D49575

llvm-svn: 337582