[clang] [flang] [libc] [llvm] [libcxx] [compiler-rt] [clang-tools-extra] [Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (PR #77753)

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cor3ntin wrote:

Similarly, `defaultedSpecialMemberIsConstexpr` should almost always return true in c++23 ( per https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2448r2.html#pnum_34 and https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2448r2.html#pnum_33 ) and `CheckExplicitlyDefaultedComparison` should alwways set the constexpr flag in c++23 mode per https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2448r2.html#pnum_39 

Sorry I did not notice all of that earlier

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