cql3: Fix relation grammar rule
cql3: Fix relation grammar rule

There's two untranslated grammar subrules in the 'relation' rule. We
have all the necessary AST classes translated, so translate the
remaining subrules.

Refs #534.

Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@scylladb.com>

kjniemi/scylla,kjniemi/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla