Supporting handler::depends_on to track edges would make it easier for users to express USM dependencies in the record &
replay model. See intel#5626 (comment)

This could be implemented by the runtime mapping default constructed sycl::events returned by queue recording to internal nodes. When one of these sycl::events is later passed into handler::depends_on the runtime can check which node in the graph
the event is associated with, and error if it is not an event returned from a queue recording.

Whether we decide to support this mechanism or not, we should update the spec to make the interaction with handler::depends_on clear.