Fix Clang warning about inconsistent use of C++11 override

We cannot use override in the Q_OBJECT macro, so tell Clang not to warn
about our lack of use of the keyword in the three virtuals that Q_OBJECT
overrides when the user uses Q_DECL_OVERRIDE in their own functions.

We can't use Q_DECL_OVERRIDE for two reasons:
 1) because that would trigger the warning in any and all virtual
    overrides the user may have in their own class (and most of Qt)
 2) because when Q_OBJECT is used in QObject, it's not overriding
    anything -- it's the base declaration

For #2, we could add a different macro exclusively for QObject that moc
would expand, but it's unnecessary work.

Change-Id: Ic9cb7896f93f06f4e86b6a42dc08f96d257f7550
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qobjectdefs.h

@@ -138,15 +138,24 @@ inline void qYouForgotTheQ_OBJECT_Macro(T1, T2) {}
 # define Q_DECL_HIDDEN_STATIC_METACALL Q_DECL_HIDDEN
 #endif
 
+#if defined(Q_CC_CLANG) && Q_CC_CLANG >= 306
+#  define Q_OBJECT_NO_OVERRIDE_WARNING      QT_WARNING_DISABLE_CLANG("-Winconsistent-missing-override")
+#else
+#  define Q_OBJECT_NO_OVERRIDE_WARNING
+#endif
+
 /* qmake ignore Q_OBJECT */
 #define Q_OBJECT \
 public: \
     Q_OBJECT_CHECK \
+    QT_WARNING_PUSH \
+    Q_OBJECT_NO_OVERRIDE_WARNING \
     static const QMetaObject staticMetaObject; \
     virtual const QMetaObject *metaObject() const; \
     virtual void *qt_metacast(const char *); \
-    QT_TR_FUNCTIONS \
     virtual int qt_metacall(QMetaObject::Call, int, void **); \
+    QT_WARNING_POP \
+    QT_TR_FUNCTIONS \
 private: \
     Q_DECL_HIDDEN_STATIC_METACALL static void qt_static_metacall(QObject *, QMetaObject::Call, int, void **); \
     struct QPrivateSignal {};