C++: fix highlighting type when there is using Namespace::Class

If type is not found we try to find 'using' declaration for this type.

Task-number: QTCREATORBUG-7903

Change-Id: I569db9e1a8504a5da3115ebbed2e823d5924f6ca
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/libs/cplusplus/LookupContext.h

@@ -311,6 +311,8 @@ public:
     }
 
 private:
+    QList<LookupItem> lookupByUsing(const Name *name, Scope *scope) const;
+
     // The current expression.
     Document::Ptr _expressionDocument;