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

possible_lhs_values returns set of possible values
for a column given some restrictions.

Current behaviour in case of a subscripted column
is to just ignore the subscript and treat
the restriction as if it were on just the column.

This seems wrong, or at least confusing,
but I won't change it in this patch to preserve the existing behaviour.

Trying to change this to something more reasonable
breaks other code which assumes that possible_lhs_values
returns a list of values.
(See partition_ranges_from_EQs() in cql3/restrictions/statement_restrictions.cc)

Signed-off-by: Jan Ciolek <b66a6f3989e26cb552db14b7ab163c3a50292e24@scylladb.com>

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