var_is_nonnullable() fails to handle invalid NOT NULL constraints

While fixing another bug in var_is_nonnullable(), I noticed $subject.
The NOTNULL_SOURCE_SYSCACHE code path (newly added for the NOT IN to
anti-join transformation) checks pg_attribute.attnotnull, which can be
true even for invalid (NOT VALID) NOT NULL constraints.

The consequence is that query_outputs_are_not_nullable() could wrongly
conclude that a subquery's output is non-nullable, causing NOT IN to
be incorrectly converted to an anti-join.

The attached fix checks the attnullability field in the relation's
tuple descriptor instead, which correctly distinguishes valid from
invalid constraints.  This is also consistent with what we do in
get_relation_notnullatts().

It could be argued that the added table_open/table_close call is a
performance concern, but I don't think so:

1. The relation is already locked by the rewriter, so
table_open(rte->relid, NoLock) is just a relcache lookup.

2. This code path is only reached when converting NOT IN to an
anti-join, and only after the outer side of the test expression has
already been proved non-nullable.

3. It is only called for relation RTEs in the subquery.

Thoughts?

- Richard