OBJECT_ATTRIBUTE is useless (or: ALTER TYPE vs ALTER TABLE for composites)

I found that the OBJECT_ATTRIBUTE symbol is useless.  I can just remove
it and replace it with OBJECT_COLUMN, and everything continues to work;
no test fails that I can find.

I thought we had a prohibition against ALTER TABLE when used on
composites, but it's not as severe as I thought.  The following commands
fail in master:

ALTER TABLE comptype RENAME TO comptype2;    -- HINT: Use ALTER TYPE
ALTER TABLE comptype SET SCHEMA sch;        -- HINT: Use ALTER TYPE

However, the following command works in master:

ALTER TABLE comptype RENAME COLUMN a TO b;
and has the same effect as this:
ALTER TYPE comptype RENAME ATTRIBUTE a TO b;

The RENAME ATTTRIBUTE case in RenameStmt is the only thing currently
using OBJECT_ATTRIBUTE; therefore, since in precisely that case we do
not prohibit using ALTER TABLE, we can just remove OBJECT_ATTRIBUTE
completely.  That leads to the attached patch, which changes no test
result at all.

This symbol was added in

commit e440e12c562432a2a695b8054964fb34e3bd823e
Author: Peter Eisentraut <peter_e@gmx.net>
Date:   Sun Sep 26 14:41:03 2010 +0300

    Add ALTER TYPE ... ADD/DROP/ALTER/RENAME ATTRIBUTE

    Like with tables, this also requires allowing the existence of
    composite types with zero attributes.

    reviewed by KaiGai Kohei

Thoughts?

--
Álvaro Herrera                http://www.2ndQuadrant.com/
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