Now with concepts support merged and mostly complete, we do not need -fconcepts-ts
(which was also misleading as we were not implementing the TS) and can enable
concepts features under C++2a. A warning will be generated if users still attempt
to use -fconcepts-ts.
Implement mangling for CSEs to match regular template-ids.
Reviewed as part of D41569 <https://reviews.llvm.org/D41569>.
Re-commit fixing failing test.
llvm-svn: 375063