Add wxDEPRECATED_MSG() and use it in a couple of places.

This macro should be used instead of wxDEPRECATED() for the new deprecations
as it allows to give a helpful explanatory message (if supported by the
compiler) and also is simpler to use as it doesn't require wrapping the entire
declaration in it but can be simply used before it.

Also add wxDEPRECATED() support for clang as a side effect.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74754 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/app.h

@@ -658,10 +658,9 @@ public:
     virtual void SetActive(bool isActive, wxWindow *lastFocus);
 
 #if WXWIN_COMPATIBILITY_2_6
-    // OBSOLETE: don't use, always returns true
-    //
     // returns true if the program is successfully initialized
-    wxDEPRECATED( bool Initialized() );
+    wxDEPRECATED_MSG("always returns true now, don't call")
+    bool Initialized();
 #endif // WXWIN_COMPATIBILITY_2_6
 
 protected: