[demangler] Resync demangler sources

Recent commits changed llvm/include/llvm/Demangle without also
changing libcxxabi/src/Demangle, which is the canonical source
location.  This resyncs those commits to the libcxxabi directory.

Commits:
* 1f9e18b6
* f53d3598
* 065044c4

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D117990.diff