fix 'ERROR: could not identify a comparison function for type unknown'

Hi, all:

when I execute simple sql,report ERROR:

postgres=# CREATE TABLE test_v(id int,name varchar(30));
CREATE TABLE
postgres=# insert into test_v values(9,'abc'),(9,'def'),(9,'gh'), (9,'gh');
INSERT 0 4
postgres=# explain (costs off) select distinct (id,name,'D3Q84xpymM',123,'123') from test_v;
                         QUERY PLAN                          
-------------------------------------------------------------
 Unique
   ->  Sort
         Sort Key: (ROW(id, name, 'D3Q84xpymM', 123, '123'))
         ->  Seq Scan on test_v
(4 rows)

postgres=# select distinct (id,name,'D3Q84xpymM',123,'123') from test_v;
ERROR:  could not identify a comparison function for type unknown

PostgreSQL  could not identify 'D3Q84xpymM' and '123' datatype:

we can allow an UNKNOWN type to change it to TEXT, 
please check my patch.

Thanks!