Fix omission of OCLASS_TRANSFORM in object_classes[]

Fix omission of OCLASS_TRANSFORM in object_classes[]

This was forgotten in cac76582053e (and its fixup ad89a5d115).  Since it
seems way too easy to miss this, this commit also introduces a mechanism
to enforce that the array is consistent with the enum.

Problem reported independently by Robert Haas and Jaimin Pan.
Patches proposed by Jaimin Pan, Jim Nasby, Michael Paquier and myself,
though I didn't use any of these and instead went with a cleaner
approach suggested by Tom Lane.

Backpatch to 9.5.

Discussion:
https://www.postgresql.org/message-id/CA+Tgmoa6SgDaxW_n_7SEhwBAc=mniYga+obUj5fmw4rU9_mLvA@mail.gmail.com
https://www.postgresql.org/message-id/29788.1437411581@sss.pgh.pa.us