We would assign the incorrect DeclContext when transforming the RequiresExprBodyDecl, causing incorrect
handling of 'this' inside RequiresExprBodyDecls (bug #45162).
Assign the current context as the DeclContext of the transformed decl.
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 support for C++2a requires-expressions.
Re-commit after compilation failure on some platforms due to alignment issues with PointerIntPair.
Differential Revision: https://reviews.llvm.org/D50360