Straight forward FastDemangle replacement in SubsPrimitiveParmItanium

Summary:
Removing FastDemangle will greatly reduce maintenance efforts. This patch replaces the last point of use in LLDB. Semantics should be kept intact.

Once this is agreed upon, we can:
* Remove the FastDemangle sources
* Add more features e.g. substitutions in template parameters, considering all variations, etc.

Depends on LLVM patch https://reviews.llvm.org/D50586

Reviewers: erik.pilkington, friss, jingham, JDevlieghere

Subscribers: kristof.beyls, chrib, lldb-commits

Differential Revision: https://reviews.llvm.org/D50587

llvm-svn: 339583