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

[via cfe-commits]

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@@ -483,6 +483,13 @@ bool Sema::CheckConstraintSatisfaction(
         *this, nullptr, ConstraintExprs, ConvertedConstraints,
         TemplateArgsLists, TemplateIDRange, OutSatisfaction);
   }
+  // Invalid templates could make their way here. Substituting them could result
+  // in dependent expressions.
+  // FIXME: Say something in the diagnostic note?
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cor3ntin wrote:

I don't think we need that fixme. It would be weird to say "note: because foo is invalid" - we usually don;t do that and it's not useful unless we can explain _why_ it is invalid

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