Fix failing assertion when trying to assign to an id referenced QML object

References to id addressed QML objects are member expressions, which are unlike
other member expressions by not being lvalues. Handle this correctly.

Task-Number: QTBUG-34890

Change-Id: Ied6230edbc561128ad36bf0d1a1918185204deec
Reviewed-by: Lars Knoll <lars.knoll@digia.com>