Add DISTINCT to GROUP BY

If these duplicates are undesirable, they can be removed using the DISTINCT clause directly on the GROUP BY. Therefore:

`GROUP BY DISTINCT ROLLUP (a, b), ROLLUP (a, c)`

is equivalent to

```
GROUP BY GROUPING SETS (
 (a, b, c),
 (a, b),
 (a, c),
 (a),
 ()
)
```
