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

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cor3ntin wrote:

> Thank you work on this! I really like this feature. Will this PR also provide support for the new constant interpreter? The new constant interpreter seems to already support some unknown constexpr features (FYI https://godbolt.org/z/xTYhGEfxT). It has the concept of `DummyValue`.

I think that can be a separate PR @tbaederr 

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