[clang] [Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (PR #77753)

[Mariya Podchishchaeva via cfe-commits]

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@@ -153,16 +153,16 @@ namespace dr1460 { // dr1460: 3.5
   namespace Defaulted {
     union A { constexpr A() = default; };
     union B { int n; constexpr B() = default; };
-    // cxx11-17-error at -1 {{defaulted definition of default constructor is not constexpr}}
+    // cxx11-17-error at -1 {{defaulted definition of default constructor that marked constexpr but never produces a constant expression is a C++23 extension}}
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Fznamznon wrote:

In C++20 mode such constructor is considered as a valid and producing `constexpr` due to https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1331r2.pdf , so no extension warning there.

https://github.com/llvm/llvm-project/pull/77753