[clang] [Clang][CodeGen] Bail out on constexpr unknown values in ConstantEmitter (PR #127525)

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https://github.com/cor3ntin approved this pull request.

LGTM modulo nit.
No changelog needed, this sould be backported to clang 20, it's a fairly severe regression.

I wonder if there are other places where we let constexpr-unknown references escapes and if there are cases where they should not be formed to begin with.

https://github.com/llvm/llvm-project/pull/127525