Port some fallthrough comments to Q_FALLTHROUGH

Change-Id: I70dd492d5c8d198589bfd642db63182cf17b133f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/plugins/platforms/xcb/qxcbsessionmanager.cpp

@@ -242,7 +242,7 @@ static void sm_performSaveYourself(QXcbSessionManager *sm)
         sm->appCommitData();
         if (sm_isshutdown && sm_cancel)
             break; // we cancelled the shutdown, no need to save state
-    // fall through
+    Q_FALLTHROUGH();
     case SmSaveLocal:
         sm->appSaveState();
         break;

src/plugins/sqldrivers/mysql/qsql_mysql.cpp

@@ -1451,7 +1451,7 @@ QString QMYSQLDriver::formatValue(const QSqlField &field, bool trimStrings) cons
             } else {
                 qWarning("QMYSQLDriver::formatValue: Database not open");
             }
-            // fall through
+            Q_FALLTHROUGH();
         default:
             r = QSqlDriver::formatValue(field, trimStrings);
         }

src/plugins/sqldrivers/odbc/qsql_odbc.cpp

@@ -1676,7 +1676,7 @@ bool QODBCResult::exec()
                                           ind);
                     break;
                 }
-            // fall through
+            Q_FALLTHROUGH();
             default: {
                 QByteArray &ba = tmpStorage[i];
                 if (*ind != SQL_NULL_DATA)
@@ -1769,7 +1769,7 @@ bool QODBCResult::exec()
                     }
                     break;
                 }
-                // fall through
+                Q_FALLTHROUGH();
             default: {
                 if (bindValueType(i) & QSql::Out)
                     values[i] = tmpStorage.at(i);