Fix crashes when cyclic inheritance

More complex situation.
Base class is a template and derived class is a template.

Task-number: QTCREATORBUG-8072
Task-number: QTCREATORBUG-7962
Change-Id: I7e8aa727fccd2bd4a779e6fe781553cd12196682
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/libs/cplusplus/LookupContext.h

@@ -58,7 +58,15 @@ public:
     }
     bool contains(const T *item)
     {
-        return _container.contains(item);
+        if (_container.contains(item))
+            return true;
+
+        foreach (const T *existingItem, _container) {
+            if (existingItem->isEqualTo(item))
+                return true;
+        }
+
+        return false;
     }
 
     void clear(const T *item)