Add model validation for constructor summary models

Author: owen-mc

csharp/ql/lib/semmle/code/csharp/dataflow/internal/ExternalFlow.qll

@@ -239,12 +239,23 @@ module ModelValidation {
     )
   }
 
+  string getIncorrectConstructorSummaryOutput() {
+    exists(string namespace, string type, string name, string output |
+      summaryModel(namespace, type, _, name, _, _, _, output, _, _, _) and
+      type = name and
+      output.matches("ReturnValue%") and
+      result =
+        "Constructor model for " + namespace + "." + type +
+          " should use `Argument[this]` in the output, not `ReturnValue`."
+    )
+  }
+
   /** Holds if some row in a MaD flow model appears to contain typos. */
   query predicate invalidModelRow(string msg) {
     msg =
       [
         getInvalidModelSignature(), getInvalidModelInput(), getInvalidModelOutput(),
-        KindVal::getInvalidModelKind()
+        getIncorrectConstructorSummaryOutput(), KindVal::getInvalidModelKind()
       ]
   }
 }