sql: allow casting a polymorphic routine parameter

[DrewKimball]

Since there is no cast from polymorphic types to other types (for example, TEXT), CRDB currently returns an error during function creation for a case like the following:

root@localhost:26257/defaultdb> CREATE FUNCTION f(x ANYELEMENT) RETURNS TEXT AS $$ SELECT x::TEXT; $$ LANGUAGE SQL;
ERROR: invalid cast: anyelement -> string
SQLSTATE: 42846

Postgres, on the other hand, allows the function to be created and called:

postgres=# CREATE FUNCTION f(x ANYELEMENT) RETURNS TEXT AS $$ SELECT x::TEXT; $$ LANGUAGE SQL;
CREATE FUNCTION
postgres=# SELECT f('hello world'::TEXT);
      f
-------------
 hello world
(1 row)

We should fix this, either by propagating some context through SemaContext to make type-checking ignore polymorphic types in this case, or by always allowing casts from polymorphic types.

Jira issue: CRDB-38387