Introduce Q_FALLTHROUGH()

Silence g++ 7.X warnings.

Change-Id: I9d06d04b496c9ec060e13e1be6f43d8fbadb1f3b
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/qmljs/qmljsevaluate.cpp

@@ -444,7 +444,7 @@ bool Evaluate::visit(AST::BinaryExpression *ast)
     //case QSOperator::And: // ### enable once implemented below
     //case QSOperator::Or:
         lhs = value(ast->left);
-        // fallthrough
+        Q_FALLTHROUGH();
     case QSOperator::Assign:
         rhs = value(ast->right);
         break;