Support parenthesized syntax for CLUSTER without a table name.

Support parenthesized syntax for CLUSTER without a table name.

b5913f6120 added a parenthesized syntax for CLUSTER, but it
requires specifying a table name.  This is unlike commands such as
VACUUM and ANALYZE, which do not require specifying a table in the
parenthesized syntax.  This change resolves this inconsistency.
This is preparatory work for a follow-up commit that will move the
unparenthesized syntax to the "Compatibility" section of the
CLUSTER documentation.

Reviewed-by: Melanie Plageman, Michael Paquier
Discussion: https://postgr.es/m/CAAKRu_bc5uHieG1976kGqJKxyWtyQt9yvktjsVX%2Bi7NOigDjOA%40mail.gmail.com