Revert "[Clang][C++23] Implement P2448R2 ..." (#85136)

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

This reverts commit 99500e8c because it
causes a behavior change for std=c++20. See
https://github.com/llvm/llvm-project/pull/77753.