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

[Tony Guillot via cfe-commits]

to268 wrote:

After merging this PR, you need to follow the documentation on [how to backport a commit to the release branch](https://llvm.org/docs/GitHub.html#releases).
Do you need this PR to be merged on your behalf?

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