[clang] [clang] Warn about deprecated volatile-qualified return types (PR #137899)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Can you add more details in the summary. Specifically your approach to fixing the issue.

Something along the lines of "In GetFullTypeForDeclarator ... moved check ... b/c ..."

https://github.com/llvm/llvm-project/pull/137899