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

[via cfe-commits]

cor3ntin wrote:

>  Is it something I can actually do myself? Or can only maintainers merge?

Yes, only maintainers can do it (you can become a maintainer after you make a few contributions)
https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access

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