Refs #18012 -- Made proxy and concrete model reverse fields consistent.

Prior to this change proxy models reverse fields didn't include the
reverse fields pointing to their concrete model.

tests/model_meta/models.py

@@ -101,6 +101,10 @@ class ProxyPerson(Person):
         proxy = True
 
 
+class PersonThroughProxySubclass(ProxyPerson):
+    pass
+
+
 class Relating(models.Model):
 
     # ForeignKey to BasePerson