QmlJS: Introduce UnknownValue.

To distinguish known-to-be-undefined from a genuinely unknown value.

Change-Id: I606b4ea4d726f94553400b8950d3c0a4e76564a8
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>

src/libs/qmljs/qmljsevaluate.cpp

@@ -69,8 +69,9 @@ const Value *Evaluate::value(AST::Node *ast)
             result = _context->lookupReference(ref);
     }
 
+    // if evaluation fails, return an unknown value
     if (! result)
-        result = _valueOwner->undefinedValue();
+        result = _valueOwner->unknownValue();
 
     return result;
 }