[clang] [concepts] Set up an instantiation scope for constraint expression comparison (PR #79698)

[Erich Keane via cfe-commits]

================
@@ -797,6 +797,15 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
   if (Inst.isInvalid())
     return nullptr;
 
+  // Set up a dummy 'instantiation' scope in the case of reference to function
+  // parameters that the surrounding function hasn't been instantiated yet. Note
+  // this may happen while we're comparing two templates' constraint
+  // equivalence.
+  LocalInstantiationScope ScopeForParameters(S);
----------------
erichkeane wrote:

I could swear I did this at one point and it caused significant problems... I don't have a problem with it as-is, but we need to make sure this passes all the CI (including the libcxx tests)

https://github.com/llvm/llvm-project/pull/79698