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

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Sirraide wrote:

Oh yeah, two more things I was going to point out: we should definiltely have some tests in unevaluated contexts (e.g. `sizeof`, `decltype`, etc.), and this also needs a release note (in `clang/docs/ReleaseNotes.rst`, probably in the section ‘Improvements to Clang’s Diagnostics).

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