Since commit a2e35b53, most CREATE and ALTER commands return the
ObjectAddress of the affected object.  This is useful for event triggers
to try to figure out exactly what happened.  This patch extends this
idea a bit further to cover ALTER TABLE as well: an auxiliary
ObjectAddress is returned for each of several subcommands of ALTER
TABLE.  This makes it possible to decode with precision what happened
during execution of any ALTER TABLE command; for instance, which
constraint was added by ALTER TABLE ADD CONSTRAINT, or which parent got
dropped from the parents list by ALTER TABLE NO INHERIT.

As with the previous patch, there is no immediate user-visible change
here.

This is all really just continuing what c504513f started.

Reviewed by Stephen Frost.