We really ought to make StdRdOptions and the other decoded forms of
reloptions self-identifying, but for the moment, assume that only plain
relations could possibly be user_catalog_tables.  Fixes problem with bogus
"ON CONFLICT is not supported on table ... used as a catalog table" error
when target is a view with cascade option.

Discussion: <26681.1477940227@sss.pgh.pa.us>