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

[Richard Smith via cfe-commits]

zygoloid wrote:

Looks like a bug to me. This warning presumably ought to be checking whether it's valid to read / write to the given location using something like [`isUserWritingOffTheEnd`](https://github.com/llvm/llvm-project/blob/b2e2ae8702e9fbbe0ef0eb5929aa2d5dc867b1c5/clang/lib/AST/ExprConstant.cpp#L12699).

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