[clang] [Clang] Implement P2280R4 Using unknown pointers and references in constant expressions (PR #95474)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

Is passing `-fstrict-flex-arrays=1` supposed to silence the warning? I still see the warning: https://godbolt.org/z/YEKhW19nK

https://github.com/llvm/llvm-project/pull/95474