[clang] [CIR] Fix missing return value warning in maybePromoteBoolResult (PR #142673)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor approved this pull request.

@AmrDeveloper posted a different fix for this here: https://github.com/llvm/llvm-project/pull/142591

I kind of like the change here better because it won't silently return an arbitrary value in release builds, and this really should be unreachable.

https://github.com/llvm/llvm-project/pull/142673