Support Redshift HLL functions/ approx_count_distinct

**Is your feature request related to a problem? Please describe.**
I don't see a reason why these can't be supported
- https://docs.aws.amazon.com/redshift/latest/dg/hyperloglog-functions.html
- https://docs.aws.amazon.com/redshift/latest/dg/r_COUNT.html

```
APPROXIMATE COUNT ( DISTINCT expression )
```

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
I'd like to use `APPROX_COUNT_DISTINCT` without breaking 😄