[BUG] row_lexicographic_comparator only works when left and right are the same table

[trevorsm7]

Describe the bug
The row_lexicographic_comparator in cudf/table/row_operators.cuh swaps the left and right indices when the column order is descending, but this only works when the same table is used for left and right.

      if (not ascending) {
        thrust::swap(lhs_index, rhs_index);
      }

Expected behavior
The column order should instead determine how the weak ordering returned by the comparator is handled.

[trevorsm7]

There don't appear to be any tests for row_lexicographic_comparator or for sorted_order (which uses it), so it would be good to add some for verifying this issue.