[clang] [OpenMP] Fix crash with extern reference and diagnose loop variable reuse in collapsed loops. (PR #203252)

[Alexey Bataev via cfe-commits]

================
@@ -8462,6 +8466,18 @@ bool OpenMPIterationSpaceChecker::checkAndSetInit(Stmt *S, bool EmitDiags) {
     }
   }
 
+  // Helper lambda to check if a loop variable is already used in an outer
+  // loop.
+  auto CheckLoopVarReuse = [&](ValueDecl *LoopVar, SourceLocation Loc) -> bool {
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alexey-bataev wrote:

Do we have a test like:
```
for (int &r = ...; r < N; ++r)
  for (r = 0; r < M; ++r)  // reuse through same canonical decl
```
Will this be diagnosed?

https://github.com/llvm/llvm-project/pull/203252