Handle GenericForeignKey class typeinfo lookup failure. (#597)

This addresses an obscure crash we're getting when defining a GenericForeignKey
subclass on a model.

Not sure how this slipped through type checking since
`helpers.lookup_class_typeinfo -> Optional[TypeInfo]` while
`.get_private_descriptor_type(type_info: TypeInfo)` so this should be a clear
type violation.

tests/typecheck/fields/test_generic_foreign_key.yml

@@ -18,4 +18,27 @@
                 class User(models.Model):
                     pass
                 class Tag(models.Model):
-                    content_object = fields.GenericForeignKey()
+                    content_object = fields.GenericForeignKey()
+-   case: generic_foreign_key_subclass_could_point_to_any_model_and_is_always_optional
+    main: |
+        from myapp.models import Tag, User
+        myuser = User()
+        Tag(content_object=None)
+        Tag(content_object=myuser)
+        Tag.objects.create(content_object=None)
+        Tag.objects.create(content_object=myuser)
+        reveal_type(Tag().content_object)  # N: Revealed type is 'Union[Any, None]'
+    installed_apps:
+        - myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models.py
+            content: |
+                from django.db import models
+                from django.contrib.contenttypes import fields
+                class User(models.Model):
+                    pass
+                class Tag(models.Model):
+                    content_object = fields.GenericForeignKey()
+                # Simulate a GenericForeignKey subclass without type infos.
+                Tag.content_object.__class__ = type('MyGenericForeignKey', (fields.GenericForeignKey,), {})