Forbid generated columns in FOR PORTION OF

Forbid generated columns in FOR PORTION OF

With virtual generated columns there is no column to assign to, and we
shouldn't assign directly to stored generated columns either.  (Once
we have PERIODs, we will allow a stored generated column here, but we
will assign to its start/end inputs.)

We can't do this in parse analysis, because views haven't yet been
rewritten, so they mask generated columns.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/agOOykf2HV26yVfU%40nathan