**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. 



{
    auto convert = [&](auto x)
    {
        using CurIndexType = decltype(x);
        auto & data = getPositionsData<CurIndexType>();
        if (sizeof(CurIndexType) > sizeof(IndexType))
            throw Exception("Converting indexes to smaller type: from " + toString(sizeof(CurIndexType)) +
                            " to " + toString(sizeof(IndexType)), ErrorCodes::LOGICAL_ERROR);
        if (sizeof(CurIndexType) != sizeof(IndexType))
        {
            size_t size = data.size();
            auto new_positions = ColumnVector<IndexType>::create(size);
            auto & new_data = new_positions->getData();
            
            for (size_t i = 0; i < size; ++i)
                new_data[i] = data[i];
            positions = std::move(new_positions);
            size_of_type = sizeof(IndexType);
        }
    };
    callForType(std::move(convert), size_of_type);
    checkSizeOfType();
}
void ColumnLowCardinality::Index::expandType()