cql3: expr: possible_lhs_values: Handle subscript
cql3: expr: possible_lhs_values: Handle subscript

This commit makes subscript an invalid argument to possible_lhs_values.
Previously this function simply ignored subscripts
and behaved as if it was called on the subscripted column
without a subscript.

This behaviour is unexpected and potentially
dangerous so it would be better to forbid
passing subscript to possible_lhs_values entirely.

Trying to handle subscript correctly is impossible
without refactoring the whole function.
The first argument is a column for which we would
like to know the possible values.
What are possible values of a subscripted column c where c[0] = 1?
All lists that have 1 on 0th position?

If we wanted to handle this nicely we would have to
change the arguments.
Such refectoring is best left until the time
when this functionality is actually needed,
right now it's hard to predict what interface
will be needed then.

Signed-off-by: cvybhu <jan.ciolek@scylladb.com>

Closes #10228

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla