QmlJS: Change from QMap to QHash for QString keys.

Tests indicate that lookup in QHash<QString, ...> is more performant
even for short strings.

Change-Id: I8e5a73d0f1a72e202522f397b16901887f9c3591
Reviewed-on: http://codereview.qt-project.org/5070
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>

src/libs/qmljs/qmljscheck.cpp

@@ -264,7 +264,7 @@ protected:
         ReturnOrThrow = 3
     };
     State _state;
-    QMap<QString, Node *> _labels;
+    QHash<QString, Node *> _labels;
     QSet<Node *> _labelledBreaks;
 
     virtual void onUnreachable(Node *)