When we invented arrays of composite types (commit bc8036fc),
we excluded system catalogs, basically just on the grounds of not
wanting to bloat pg_type.  However, it's definitely inconsistent that
catalogs' composite types can't be put into arrays when others can.
Another problem is that the exclusion is done by checking
IsUnderPostmaster in heap_create_with_catalog, which means that

(1) If a user tries to create a table in single-user mode, it doesn't
get an array type.  That's bad in itself, plus it breaks pg_upgrade.

(2) If someone drops and recreates a system view or information_schema
view (as we occasionally recommend doing), it will now have an array
type where it did not before, making for still more inconsistency.

So this is all pretty messy.  Let's just get rid of the inconsistency
and decree that system-created relations should have array types if
similar user-created ones would, i.e. it only depends on the relkind.
As of HEAD, that means that the initial contents of pg_type grow from
411 rows to 605, which is a lot of growth percentage-wise, but it's
still quite a small catalog compared to others.

Wenjing Zeng, reviewed by Shawn Wang, further hacking by me

Discussion: https://postgr.es/m/761F1389-C6A8-4C15-80CE-950C961F5341@gmail.com