whenAll() and whenAny() create a shared context object which is
referenced by the continuation lambda. The refcount of context is only
correctly managed when it is copied non-const to the lambda's
capture list.
Fixes: QTBUG-116731
Pick-to: 6.6 6.6.0
Change-Id: I8e79e1a0dc867f69bbacf1ed873f353a18f6ad38
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>