[clang] [Sema] Diagnose use of if/else-if condition variable inside else-if/else branch(s) (PR #156436)

[Nikita Popov via cfe-commits]

nikic wrote:

> CC @nikic could you put this on llvm-compile-time-tracker so we know what the overhead of this is if any?

Not seeing an impact when applying this patch -- but that's probably expected as the warning is not enabled by default? I'd probably additional changes to enable it.

https://github.com/llvm/llvm-project/pull/156436