[clang] Force an ICE when reentrantly reading a concept decl (PR #224677)

[Ian Petersen via cfe-commits]

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@@ -3249,7 +3249,14 @@ class ImplicitConceptSpecializationDecl final
     : public Decl,
       private llvm::TrailingObjects<ImplicitConceptSpecializationDecl,
                                     TemplateArgument> {
-  unsigned NumTemplateArgs;
+  unsigned NumTemplateArgs : 31;
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ispeters wrote:

Oh, I thought that might be it, but I feel like that'd be kind of brittle because `ArgsPopulated` would only exist when `!defined(NDEBUG)`. I suppose it could be initialized at the point of declaration and the assignment could also be guarded by `#ifndef DEBUG`, but it just feels messy.

https://github.com/llvm/llvm-project/pull/224677