[clang] [C2y] Correctly handle incomplete types in generic selections (PR #141596)

[Erich Keane via cfe-commits]

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@@ -1874,9 +1874,11 @@ ExprResult Sema::CreateGenericSelectionExpr(
 
         if (D != 0) {
           Diag(Types[i]->getTypeLoc().getBeginLoc(), D)
-            << Types[i]->getTypeLoc().getSourceRange()
-            << Types[i]->getType();
-          TypeErrorFound = true;
+              << Types[i]->getTypeLoc().getSourceRange() << Types[i]->getType();
+          if (getDiagnostics().getDiagnosticLevel(
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erichkeane wrote:

As a nit... this sort feels like the thing we should be asking the diagnostic builder, or at least the Diagnostics Engine.   

Also, kinda showing the 'downside' to the `save the diagnostic id, then print it later` thing here.

https://github.com/llvm/llvm-project/pull/141596