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

[Mariya Podchishchaeva via cfe-commits]

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@@ -165,6 +165,8 @@ C++23 Feature Support
 - Added a separate warning to warn the use of attributes on lambdas as a C++23 extension
   in previous language versions: ``-Wc++23-lambda-attributes``.
 
+- Implemented `P2448R2: Relaxing some constexpr restrictions <https://wg21.link/P2448R2>`_.
+
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Fznamznon wrote:

Okay, done. I'm not sure I updated `LanguageExtensions.rst`, please take a look.
I also added some testing for enabling extension in earlier language modes.

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