[clang] [Clang][Sema] Fix crash in CheckUsingDeclQualifier due to diagnostic missing an argument (PR #161277)

[Erich Keane via cfe-commits]

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@@ -225,9 +226,12 @@ void TrivialButNonPODThroughEllipsis() {
   Ellipsis(1, TrivialButNonPOD()); // expected-warning {{passing object of trivial but non-POD type 'TrivialButNonPOD' through variadic function is incompatible with C++98}}
 }
 
+// FIXME I think we generate this diagnostic in C++20
+#ifndef CXX20COMPAT
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erichkeane wrote:

Instead of the macro, just make this a NOTCXX20-warning

https://github.com/llvm/llvm-project/pull/161277