[1.11.x] Fixed #28038 -- Restored casting to text of builtin lookups on PostgreSQL.

Reverted 658f1e8 which broke code using __icontains's implicit cast to ::text
on ArrayField.

Thanks Peter J. Farrell for the report.

Backport of a354c69055 from master

docs/releases/1.11.1.txt

@@ -12,3 +12,6 @@ Bugfixes
 * Made migrations respect ``Index``’s ``name`` argument. If you created a
   named index with Django 1.11, ``makemigrations`` will create a migration to
   recreate the index with the correct name (:ticket:`28051`).
+
+* Fixed a crash when using a ``__icontains`` lookup on a ``ArrayField``
+  (:ticket:`28038`).