While in SQL DISTINCT applies to the result set, in CQL it applies
to the table being selected, and doesn't allow GROUP BY with clustering
keys. So reject the combination like Cassandra does.
While this is not an important issue to fix, it blocks un-xfailing
other issues, so I'm clearing it ahead of fixing those issues.
An issue is unmarked as xfail, and other xfails lose this issue
as a blocker.
Fixes#12479Closes#14970