Fixed #29896 -- Fixed incorrect Model.save() cache relation clearing for foreign keys that use to_field.

Regression in ee49306176.

tests/many_to_one/tests.py

@@ -666,3 +666,9 @@ class ManyToOneTests(TestCase):
         self.a.reporter_id = self.r2.pk
         self.a.save()
         self.assertEqual(self.a.reporter, self.r2)
+
+    def test_cached_foreign_key_with_to_field_not_cleared_by_save(self):
+        parent = Parent.objects.create(name='a')
+        child = ToFieldChild.objects.create(parent=parent)
+        with self.assertNumQueries(0):
+            self.assertIs(child.parent, parent)