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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> That said, this patch might be worth having in that branch AS WELL to cover any other missed cases. I would be OK having this patch HERE only on that branch or replaced with an assert in main.

#128409 should cover all the cases this patch would cover.  I'm a little concerned #128409 still isn't broad enough... but not in a way that would trigger this check.  I'll continue discussion there.

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