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

[Amy Huang via cfe-commits]

amykhuang wrote:

I looked at this a bit and the change that increases the constexpr restrictions is that `ext_defaulted_comparison_constexpr_mismatch` (which was added in https://reviews.llvm.org/D146090) became `err_incorrect_defaulted_comparison_constexpr`?

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