add GenericForeignKey support, remove some false-positives

test-data/typecheck/fields/test_generic_foreign_key.yml

@@ -0,0 +1,22 @@
+-   case: generic_foreign_key_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:
+        - django.contrib.contenttypes
+        - 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()

test-data/typecheck/fields/test_related.yml

@@ -381,10 +381,10 @@
         -   path: myapp/models.py
             content: |
                 from django.db import models
-                class Book(models.Model):
-                    publisher = models.ForeignKey(to='Publisher', on_delete=models.CASCADE)
                 class Publisher(models.Model):
                     pass
+                class Book(models.Model):
+                    publisher = models.ForeignKey(to='Publisher', on_delete=models.CASCADE)
 
 -   case: test_foreign_key_field_without_backwards_relation
     main: |