[AST] Fix the incorrect auto-keyword loc for constrained auto type loc.

E.g.  `Concept auto Func();`

The nameLoc for the constained auto type loc pointed to the concept name
loc, it should be the auto token loc. This patch fixes it, and remove
a relevant hack in clang-tidy check.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D117009