**Describe the bug**
When using ORDER BY with multiple expressions on a MergeTree table, the ordering is not correct.


**How to reproduce**
` yandex/clickhouse-server:20.5.2.7`

Create the table:
```sql
create table order_test
(
    timestamp DateTime64(3),
    color     LowCardinality(Nullable(String))
) engine = MergeTree()
      PARTITION BY toYYYYMM(timestamp)
      ORDER BY timestamp
      SETTINGS index_granularity = 8192;
```

Fill the table (run a few time times to get a range of timestamps):
```sql
insert into order_test
select now64(),
       arrayElement(['red', 'green', 'blue', null], modulo(number, 4) + 1) as color
from (select number from system.numbers limit 1000000);
```

Query:
```sql
SELECT count(),
       color,
       toStartOfSecond(timestamp) AS `second`
FROM order_test AS i
GROUP BY color, `second`
ORDER BY color, `second` desc
LIMIT 500;
```


See out of order results:
```
500000,blue,2020-08-21 18:30:06.000
500000,blue,2020-08-21 18:30:09.000
250000,blue,2020-08-21 18:30:03.000
250000,blue,2020-08-21 18:30:08.000
250000,blue,2020-08-21 18:30:07.000
```

**Expected behavior**
See results in order by descending second:
```
500000,blue,2020-08-21 18:30:09.000
250000,blue,2020-08-21 18:30:08.000
250000,blue,2020-08-21 18:30:07.000
500000,blue,2020-08-21 18:30:06.000
250000,blue,2020-08-21 18:30:03.000
```

Notes:

It appears when I take out `LowCardinality` from the color column, the order works as expected. 



{
        if (index.empty())
            return ColumnVector<T>::create();
        auto size = index.size();
        T max_val = index[0];
        for (size_t i = 1; i < size; ++i)
            max_val = std::max(max_val, index[i]);
        
        if (max_val > size)
            return mapUniqueIndexImplRef(index);
        auto map_size = UInt64(max_val) + 1;
        PaddedPODArray<T> map(map_size, 0);
        T zero_pos_value = index[0];
        index[0] = 0;
        T cur_pos = 0;
        for (size_t i = 1; i < size; ++i)
        {
            T val = index[i];
            if (val != zero_pos_value && map[val] == 0)
            {
                ++cur_pos;
                map[val] = cur_pos;
            }
            index[i] = map[val];
        }
        auto res_col = ColumnVector<T>::create(UInt64(cur_pos) + 1);
        auto & data = res_col->getData();
        data[0] = zero_pos_value;