Fix #2656: correctly transform DISTINCT with ORDER BY into DISTINCT ON

[Mytherin]

Fixes #2656. We choose to mimick SQLite's behavior here and transform the query into a DISTINCT ON with the queries that are mentioned in the SELECT clause, i.e. the following two queries are identical:

CREATE TABLE T (t1 int, t2 int);
INSERT INTO t VALUES (1, 1), (1, 2);

SELECT DISTINCT t1
FROM T
ORDER BY t1, t2;

SELECT t1 FROM (
  SELECT DISTINCT ON(t1) t1, t2
  FROM T
  ORDER BY t1, t2
) T;