[clang] [CIR][NFS] Fix missing return statement warning (PR #142591)

[Andy Kaylor via cfe-commits]

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

@mmha also posted a fix for the maybePromoteBoolResult here: https://github.com/llvm/llvm-project/pull/142673

The removal of the dead code below the return in VisitUnaryLNot should be done as it is here, but I have a slight preference for Morris' handling of the maybePromoteBoolResult return.

https://github.com/llvm/llvm-project/pull/142591