Add __[_[_]]Z demangling to new common demangle function

This is a follow-up to r351448. It adds support for other _*Z extensions
of the Itanium demanling, to the newly available demangle function
heuristic.

Reviewed by: erik.pilkington, rupprecht, grimar

Differential Revision: https://reviews.llvm.org/D56855

llvm-svn: 351551