[clang] [Clang] Treat constexpr-unknown value as invalid in `EvaluateAsInitializer` (PR #128409)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

I get that we are allowing constexpr unknown values into codegen and that is a mistake but I don't totally follow the fix. Can you go into more details?

If I am reading the code correctly there should be some constexpr cases that generate a diagnostic, can we verify that with a test?

https://github.com/llvm/llvm-project/pull/128409