**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. 



for (size_t i = 0; i < map_size; ++i)
        {
            auto val = map[i];
            if (val)
                data[val] = static_cast<T>(i);
        }
        return res_col;
    }
    
    MutableColumnPtr mapUniqueIndex(IColumn & column)
    {
        if (auto * data_uint8 = getIndexesData<UInt8>(column))
            return mapUniqueIndexImpl(*data_uint8);
        else if (auto * data_uint16 = getIndexesData<UInt16>(column))
            return mapUniqueIndexImpl(*data_uint16);
        else if (auto * data_uint32 = getIndexesData<UInt32>(column))
            return mapUniqueIndexImpl(*data_uint32);
        else if (auto * data_uint64 = getIndexesData<UInt64>(column))
            return mapUniqueIndexImpl(*data_uint64);
        else
            throw Exception("Indexes column for getUniqueIndex must be ColumnUInt, got" + column.getName(),
                            ErrorCodes::LOGICAL_ERROR);
    }
}
ColumnLowCardinality::ColumnLowCardinality(MutableColumnPtr && column_unique_, MutableColumnPtr && indexes_, bool is_shared)
    : dictionary(std::move(column_unique_), is_shared), idx(std::move(indexes_))
{
    idx.check(getDictionary().size());
}
void ColumnLowCardinality::insert(const Field & x)