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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> We can enable it as an extension in older language modes later as a separate patch in case there is user's demand on this. @cor3ntin , @AaronBallman please let me know what you think.

Thank you for investigating this! I'm perfectly fine not enabling this as an extension in older language modes until we have a strong enough need to justify the efforts.

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