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

[via cfe-commits]

cor3ntin wrote:

As discussed in the Clang WG meeting, we think this is in good shape and should be merged this week (pending adding a release note) despite the weird warning change.

This gives us the opportunity to get some experience before the branch and fix numerous bug reports, while having time to revert if something explodes in downstreams

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