[clang] Better diagnostics when assertion fails in `consteval` (PR #130458)

[JJ Marr via cfe-commits]

jj-marr wrote:

> LGTM modulo nit.
> 
> Are you happy with the current state? @cor3ntin @Sirraide

Shouldn't CI be passing? I can't figure out why there are failures.

https://github.com/llvm/llvm-project/pull/130458