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

[Younan Zhang via cfe-commits]

zyn0217 wrote:

@cor3ntin I moved the check to `CheckConstraintSatisfaction` so we don't end up with many substitution failure notes. Can you check it out again?

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