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

[via cfe-commits]

arrowten wrote:

@erichkeane Hi Erich, please have a look. I have not used the ```ASTContext::isSameEntity()``` because it was giving out unwanted warnings for testcases 5 and 6. Please let me know if the new changes look good.

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