Reject wholerow references where ColumnRef is forbidden

Hi Hackers,

Here is a fix to some crashes when using a wholerow reference to sneak
in a ColumnRef where it shouldn't be allowed. This was originally
reported in [0] by Andres while reviewing FOR PORTION OF, but it is
actually a problem on master, going back at least to v14. You can
reach a crash like so:

-- crashes
ALTER TABLE t ADD COLUMN b int DEFAULT (ROW(t.*)).f1;

-- crashes
CREATE TABLE p1 PARTITION OF p FOR VALUES IN ((ROW(p1.*)).f1);

-- wrongly accepted
CREATE TABLE t (a int, b int DEFAULT (ROW(t.*)).f1);
ALTER TABLE t ALTER COLUMN b SET DEFAULT (ROW(t.*)).f1;
INSERT INTO t (a) VALUES (1); -- crashes here instead

Today, we forbid some EXPR_KINDs in transformColumnRef, but we need to
do that in (or near) ExpandColumnRefStar as well. I factored out the
check to checkColumnRefAllowed to avoid future inconsistencies.

[0] https://www.postgresql.org/message-id/yfers7d2kizq6ockz4nu7e7avq3gj5anpd7ilxkazyvicv4qgy@3cesow2bbhnx

Yours,

--
Paul ~{:-)
pj(at)illuminatedcomputing(dot)com