[clang] [Sema] Improve diagnostic for 'auto' in local-class member params (PR #167201)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis requested changes to this pull request.

If AI was used to create PR, please disclose it. And to what extent it was used.

Please remove unrelated changes.

Please read <https://clang.llvm.org/docs/InternalsManual.html#verifying-diagnostics>.

Please remove duplicate test.

https://github.com/llvm/llvm-project/pull/167201