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

[Erich Keane via cfe-commits]

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+// RUN: %clang_cc1 -fsyntax-only -verify -Wconditional-scope -DTEST_1 %s
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erichkeane wrote:

I gave you some ideas, dependence, non-evaluated contexts, etc.  

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