Expand joins when calculating PostgreSQL "WITH FOR UPDATE OF"

Expand joins when calculating PostgreSQL "WITH FOR UPDATE OF"

Modified the :paramref:`.Select.with_for_update.of` parameter so that if a
join or other composed selectable is passed, the individual :class:`.Table`
objects will be filtered from it, allowing one to pass a join() object to
the parameter, as occurs normally when using joined table inheritance with
the ORM.  Pull request courtesy Raymond Lu.

Fixes: #4550
Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Closes: #4551
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4551
Pull-request-sha: 452da77d154a4087d530456db1c9af207d65cef4

Change-Id: If4b7c231f7b71190d7245543959fb5c3351125a1