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

[Erich Keane via cfe-commits]

erichkeane wrote:

> @Alcaro
> 
> > I agree the worst case would be bad; if I'm reading things correctly, it'd be quadratic for things like
> 
> Any specific code changes you want to suggest?

I suggested something using the `Scope` type above: Instead of a visitor, use the scope to figure out when we're in the 'else' block, then do the diagnostic on the init-variable there.  We might need to store a slight more bit on `Scope`, but it should be much less overhead because of it.

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