Allow COMMENT ON to accept an expression rather than just a string

Hi.

There's a TODO item about making COMMENT ON accept an expression. The
grammar change is simple (SConst|NULL_P->a_expr), but as far as I can
see, there are no similar utility commands that take expressions, and
I'm not very familiar with the planner and executor, so I could use
some advice about how to evaluate the expression.

Also: what expressions should the code accept? I want to be able to use
a parameter. Does that require extra work? What about subqueries, would
they be useful in this context?

(Or would it be better to just have functions that can set object and
column descriptions instead?)

Thanks.

-- ams