This is just like commit 14f67a8e, but
for CREATE PARTITION rather than ATTACH PARTITION.

Jeevan Ladhe, with test case changes by me.

Discussion: http://postgr.es/m/CAOgcT0MWwG8WBw8frFMtRYHAgDD=tpt6U7WcsO_L2k0KYpm4Jg@mail.gmail.com