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

[Yingwei Zheng via cfe-commits]

dtcxzyw wrote:

> Should the other Expr::Evaluate* entry points have the same bailout? Do you want to do that in this patch, or a followup?

Sorry I don't have enough bandwidth to work on other potential constexpr-unknown-related issues. I just want to close https://github.com/llvm/llvm-project/issues/127475 as soon as possible.


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