sql: support pg_catalog.regclass when casting

[robins]

When casting, apparently Cockroachdb doesn't (yet) support pg_catalog.regclass although regclass works fine. It would help if CockroachDB could (eventually) support this, to avoid namespace clashes.

In Postgres

postgres2=> select 'a'::regclass::OID;
   oid
----------
 20041642
(1 row)
postgres2=> select 'a'::pg_catalog.regclass::OID;
   oid
----------
 20041642
(1 row)

In CockroachDB

bank=> \d a
                      Table "bank.a"
 Column |  Type  |     Collation     | Nullable | Default
--------+--------+-------------------+----------+---------
 b      | bigint | Not Supported Yet |          |
Indexes:
    "primary" PRIMARY KEY, (Not Supported Yet)
bank=> select 'a'::regclass::OID;
 'a'::REGCLASS::OID
--------------------
          818765073
(1 row)

bank=> select 'a'::pg_catalog.regclass::OID;
ERROR:  syntax error at or near "pg_catalog"
select 'a'::pg_catalog.regclass::OID;
            ^

Downstream issue robins/postgres#23 is dependent on this to be fixed.

[knz]

I have had a look into this. This is a larger issue than it first seemed because it requires a rather large refactor of our SQL grammar. Will look into it for 2.1 instead of 2.0.

[knz]

Proposing #23208 but lemme see if my colleagues agree on having such a special case compatibility rule.

[jordanlewis]

Closing as a duplicate of #16395.