Don't try to record dependency on a dropped column's datatype

When creating a relation with a dropped column, we called
recordDependencyOn() also on the datatype of the dropped column, which
is always InvalidOid. In versions 15 and above, that was harmless
because recordDependencyOn() considers InvalidOid as a pinned object,
and skips over it. On version 14, isPinnedObject() does not consider
InvalidOid as pinned, so we created a bogus pg_depend entry with
refobjectid == 0.

As far as I can tell, the only case when AddNewAttributeTuples() is
called with dropped columns is when performing a table-rewriting ALTER
TABLE command. That temporarily creates a new relation with the same
columns, including dropped ones, then swaps the relations, and drops
the newly created table again. So even on version 14, the bogus
pg_depend entry was only on the transient relation that was dropped at
the end of the ALTER TABLE command, which was harmless.

Even though this is harmless, let's be tidy, similar to commit
713bce9484. The reason I noticed this now and why I backported this,
is because the next commit will add code to acquire locks on the
referenced objects, and we don't want to acquire a lock on InvalidOid.

Discussion: https://postgr.es/m/ZiYjn0eVc7pxVY45@ip-10-97-1-34.eu-west-3.compute.internal
Backpatch-through: 14

src/backend/catalog/heap.c

@@ -869,6 +869,9 @@ AddNewAttributeTuples(Oid new_rel_oid,
 	{
 		Form_pg_attribute attr = TupleDescAttr(tupdesc, i);
 
+		if (attr->attisdropped)
+			continue;
+
 		/* Add dependency info */
 		ObjectAddressSubSet(myself, RelationRelationId, new_rel_oid, i + 1);
 		ObjectAddressSet(referenced, TypeRelationId, attr->atttypid);