This eliminates some more tedium in adding new catalog entries,
specifically the need to set up an array type when adding a new
built-in data type.  Now it's sufficient to assign an OID for the
array type and write it in an "array_type_oid" metadata field.
You don't have to fill the base type's typarray link explicitly, either.

No catversion bump since the contents of pg_type aren't changed.
(Well, their order might be different, but that doesn't matter.)

John Naylor, reviewed and whacked around a bit by
Dagfinn Ilmari Mannsåker, and some more by me.

Discussion: https://postgr.es/m/CAJVSVGVTb6m9pJF49b3SuA8J+T-THO9c0hxOmoyv-yGKh-FbNg@mail.gmail.com