Add support for not-null constraints on virtual generated columns

Add support for not-null constraints on virtual generated columns

This was left out of the original patch for virtual generated columns
(commit 83ea6c54025).

This just involves a bit of extra work in the executor to expand the
generation expressions and run a "IS NOT NULL" test against them.

There is also a bit of work to make sure that not-null constraints are
checked during a table rewrite.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Navneet Kumar <thanit3111@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/CACJufxHArQysbDkWFmvK+D1TPHQWWTxWN15cMuUaTYX3xhQXgg@mail.gmail.com