[clang] [Clang] Implement P2280R4 Using unknown pointers and references in constant expressions (PR #95474)

[Aaron Ballman via cfe-commits]

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@@ -249,13 +249,15 @@ class APValue {
   struct NoLValuePath {};
   struct UninitArray {};
   struct UninitStruct {};
+  struct ConstexprUnknown {};
 
   template <typename Impl> friend class clang::serialization::BasicReaderBase;
   friend class ASTImporter;
   friend class ASTNodeImporter;
 
 private:
   ValueKind Kind;
+  bool AllowConstexprUnknown = false;
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AaronBallman wrote:

Would it make sense to use a bit-field here so we can pack this bit together with the `Kind`?

https://github.com/llvm/llvm-project/pull/95474