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

[via cfe-commits]

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@@ -13594,6 +13594,9 @@ def warn_acc_var_referenced_lacks_op
               "reference has no effect">,
       InGroup<DiagGroup<"openacc-var-lacks-operation">>,
       DefaultError;
+def warn_out_of_scope_var_usage
+    : Warning<"variable %0 declared in 'if' block is either false or null">,
----------------
Sirraide wrote:

```suggestion
    : Warning<"variable %0 declared in 'if' condition is always false or null here">,
```
Also, this should use `%enum_select` and say either ‘false’ or ‘null’, depending on which one is appropriate for the type of the variable.

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