[clang] [Concepts] Avoid substituting into constraints for invalid TemplateDecls (PR #75697)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

Hmmm, actually I'm thinking about whether we can move the check to somewhere in Parser so that these don't end up in the evaluation function.

https://github.com/llvm/llvm-project/pull/75697