Fixed #29998 -- Allowed multiple OneToOneFields to the parent model.

We assumed that any OneToOneField's in a child model must be the
parent link and raised an error when parent_link=True was not
specified. This patch allows to specify multiple OneToOneField's to
the parent model.

OneToOneField's without a custom related_name will raise fields.E304
and fields.E305 so this should warn users when they try to override
the auto-created OneToOneField.

tests/migrations/test_state.py

@@ -345,6 +345,7 @@ class StateTests(SimpleTestCase):
                     'migrations.Tag',
                     models.CASCADE,
                     auto_created=True,
+                    parent_link=True,
                     primary_key=True,
                     to_field='id',
                     serialize=False,