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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Opened #129844 with a case that still fails.  Other entry-points to constant evaluation need similar treatment to EvaluateAsInitializer.

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