sql: cannot cast to pseudo types such as any and anyelement

(Thanks to @paulniziolek for finding this!)

PostgreSQL allows casts to pseudo-types such as `any`, `anyelement`, etc. I'm not sure what the utility is, but here are some examples:

```sql
SELECT 'a'::"any";
SELECT 1::"any";
SELECT 2.5::anyelement;
SELECT true::anyelement;
```

CockroachDB does not allow these casts:

```
demo@127.0.0.1:26257/demoapp/defaultdb> SELECT 'a'::"any";
ERROR: invalid cast: string -> any
SQLSTATE: 42846
demo@127.0.0.1:26257/demoapp/defaultdb> SELECT 1::"any";
ERROR: invalid cast: int -> any
SQLSTATE: 42846
demo@127.0.0.1:26257/demoapp/defaultdb> SELECT 2.5::anyelement;
ERROR: invalid cast: decimal -> anyelement
SQLSTATE: 42846
demo@127.0.0.1:26257/demoapp/defaultdb> SELECT true::anyelement;
ERROR: invalid cast: bool -> anyelement
SQLSTATE: 42846
```

Jira issue: CRDB-53103