Eng 6436 distinct reroute

It's not immediately obvious that we must have a GROUP BY clause if we arrive here, so it would be good to add a comment explaining this:
```C++
  // We only get here if we have a GROUP BY clause, since SELECT DISTINCT is converted to 
  // grouping by all columns in the select list prior to planning if no GROUP BY is present.
```
You could even convert the name of the function to `handleDistinctWithGroupBy` to make it really obvious.