Add Q_FALLTHROUGH for Qt < 5.8

... and make use of it.
With gcc 7, the new option -Wimplicit-fallthrough is introduced and
added to the -Wextra set, triggering dozens of warnings in our sources.
Therefore, we annotate all obviously intended fall-throughs. The ones
that are still left are unclear and need to be checked by the respective
maintainer.

Change-Id: I44ead33cd42a4b41c28ee5fcb5a31db272710bbc
Reviewed-by: Nikita Baryshnikov <nib952051@gmail.com>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/3rdparty/cplusplus/Parser.cpp

@@ -27,6 +27,8 @@
 #include "ObjectiveCTypeQualifiers.h"
 #include "QtContextKeywords.h"
 
+#include <utils/qtcfallthrough.h>
+
 #include <unordered_map>
 #include <utility>
 
@@ -442,7 +444,7 @@ bool Parser::skipUntilStatement()
             case T_AT_THROW:
                 if (_languageFeatures.objCEnabled)
                     return true;
-
+                Q_FALLTHROUGH();
             default:
                 consumeToken();
         }