**Apache Airflow version**:

2.0.1

**What happened**:

When clearing task across dags using `ExternalTaskMarker` the dag state of the external `DagRun` is not set to active. So cleared tasks in the external dag will not automatically start if the `DagRun` is a Failed or Succeeded state.

**What you expected to happen**:

The external `DagRun` run should also be set to Running state.

**How to reproduce it**:

Clear tasks in an external dag using an ExternalTaskMarker.


**Anything else we need to know**:

Looking at the code is has:

https://github.com/apache/airflow/blob/b23fc137812f5eabf7834e07e032915e2a504c17/airflow/models/dag.py#L1323-L1335

It seems like it intentionally calls the dag member method `set_dag_run_state` instead of letting the helper function `clear_task_insntances` set the `DagRun` state. But the member method will only change the state of `DagRun`s of dag where the original task is, while I believe `clear_task_instances` would correctly change the state of all involved `DagRun`s.

airflow/models/dag.py
"""
        Returns the number of task instances in the given DAG.
        :param session: ORM session
        :param dag_id: ID of the DAG to get the task concurrency of
        :type dag_id: unicode
        :param task_ids: A list of valid task IDs for the given DAG
        :type task_ids: list[unicode]
        :param states: A list of states to filter by if supplied
        :type states: list[state]
        :return: The number of running tasks
        :rtype: int
        """
        qry = session.query(func.count(TaskInstance.task_id)).filter(
            TaskInstance.dag_id == dag_id,
        )
        if task_ids:
            qry = qry.filter(
                TaskInstance.task_id.in_(task_ids),
            )
        if states:
            if None in states:
                if all(x is None for x in states):
                    qry = qry.filter(TaskInstance.state.is_(None))
                else:
                    not_none_states = [state for state in states if state]
                    qry = qry.filter(
                        or_(TaskInstance.state.in_(not_none_states), TaskInstance.state.is_(None))
                    )
            else:
                qry = qry.filter(TaskInstance.state.in_(states))