**Apache Airflow version**:
2.0.1

**Kubernetes version**:
Server Version: version.Info{Major:"1", Minor:"18+", GitVersion:"v1.18.9-eks-d1db3c", GitCommit:"d1db3c46e55f95d6a7d3e5578689371318f95ff9", GitTreeState:"clean", BuildDate:"2020-10-20T22:18:07Z", GoVersion:"go1.13.15", Compiler:"gc", Platform:"linux/amd64"}

**What happened**:
Clearing a failed subdag task with Downstream+Recursive does not automatically set the state of the parent dag to 'running' so that the downstream parent tasks can execute.

The work around is to manually set the state of the parent dag to running after clearing the subdag task

**What you expected to happen**:
With airflow version 1.10.4 the parent dag was automatically set to 'running' for this same scenario

**How to reproduce it**:
- Clear a failed subdag task selecting option for Downstream+Recursive
- See that all down stream tasks in the subdag as well as the parent dag have been cleared
- See that the parent dag is left in 'failed' state. 

airflow/models/dag.py
from airflow.utils.dates import cron_presets, date_range as utils_date_range
from airflow.utils.file import correct_maybe_zipped
from airflow.utils.helpers import validate_key
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import provide_session
from airflow.utils.sqlalchemy import Interval, UtcDateTime, skip_locked, with_row_locks
from airflow.utils.state import State
from airflow.utils.types import DagRunType, EdgeInfoType
if TYPE_CHECKING:
    from airflow.utils.task_group import TaskGroup
try:
    from re import Pattern as PatternType  
except ImportError:
    PatternType = type(re.compile('', 0))
log = logging.getLogger(__name__)
ScheduleInterval = Union[str, timedelta, relativedelta]
DEFAULT_VIEW_PRESETS = ['tree', 'graph', 'duration', 'gantt', 'landing_times']
ORIENTATION_PRESETS = ['LR', 'TB', 'RL', 'BT']
DagStateChangeCallback = Callable[[Context], None]
def get_last_dagrun(dag_id, session, include_externally_triggered=False):
    """
    Returns the last dag run for a dag, None if there was none.
    Last dag run can be any type of run eg. scheduled or backfilled.
    Overridden DagRuns are ignored.
    """
    DR = DagRun
    query = session.query(DR).filter(DR.dag_id == dag_id)
    if not include_externally_triggered:
        query = query.filter(DR.external_trigger == False)  
    query = query.order_by(DR.execution_date.desc())