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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Does it even make sense for evaluateValue to return a constexpr-unknown value?  If I'm following correctly, we can't actually do anything useful with such a result: we don't actually know what the value is.  It would make things simpler if we kept such APValues as an internal implementation detail of ExprConstant. 

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