Allow RECORD and RECORD[] to be specified in function coldeflists.

Allow RECORD and RECORD[] to be specified in function coldeflists.

We can't allow these pseudo-types to be used as table column types,
because storing an anonymous record value in a table would result
in data that couldn't be understood by other sessions.  However,
it seems like there's no harm in allowing the case in a column
definition list that's specifying what a function-returning-record
returns.  The data involved is all local to the current session,
so we should be just as able to resolve its actual tuple type as
we are for the function-returning-record's top-level tuple output.

Elvis Pranskevichus, with cosmetic changes by me

Discussion: https://postgr.es/m/11038447.kQ5A9Uj5xi@hammer.magicstack.net