Method return values as well as whatever we hide behind "unknown" can
contain shadowable properties. We need to check it.
Pick-to: 6.6 6.5 6.2
Task-number: QTBUG-117800
Change-Id: I518bc11fd0c9c69340bf621198eeaf4c95d17dae
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Olivier De Cannière <olivier.decanniere@qt.io>