Users queries can skip the SQL_CALC_FOUND_ROWS queries

Running a query such as the following, followed by a `SELECT FOUND_ROWS()` will result in an error, and the found rows not being returned.
```
SELECT DISTINCT SQL_CALC_FOUND_ROWS wp_users.ID FROM wp_users WHERE 1....
```

results in this error:
```
Attempted SELECT FOUND_ROWS() without prior SQL_CALC_FOUND_ROWS.
```

For an example of code that can trigger this, see https://github.com/WordPress/wporg-mu-plugins/pull/412