[clang] [Clang] Avoid querying tag definitions for invalid DeclSpecs (PR #210085)

[Tony Guillot via cfe-commits]

https://github.com/to268 commented:

I have discussed with Aaron concerning the diagnostics change.
This PR would need to restore the previous `auto` warning from C++11 and later.
[Godbolt diagnostics reference](https://godbolt.org/z/aYqYboMKW)

https://github.com/llvm/llvm-project/pull/210085