Replace pg_class.relhasexclusion with pg_index.indisexclusion.

Replace pg_class.relhasexclusion with pg_index.indisexclusion.

There isn't any need to track this state on a table-wide basis, and trying
to do so introduces undesirable semantic fuzziness.  Move the flag to
pg_index, where it clearly describes just a single index and can be
immutable after index creation.