**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. 



equal_range = std::move(new_ranges);
}
std::vector<MutableColumnPtr> ColumnLowCardinality::scatter(ColumnIndex num_columns, const Selector & selector) const
{
    auto columns = getIndexes().scatter(num_columns, selector);
    for (auto & column : columns)
    {
        auto unique_ptr = dictionary.getColumnUniquePtr();
        column = ColumnLowCardinality::create(IColumn::mutate(std::move(unique_ptr)), std::move(column));
    }
    return columns;
}
void ColumnLowCardinality::setSharedDictionary(const ColumnPtr & column_unique)
{
    if (!empty())
        throw Exception("Can't set ColumnUnique for ColumnLowCardinality because is't not empty.",
                        ErrorCodes::LOGICAL_ERROR);
    dictionary.setShared(column_unique);
}
ColumnLowCardinality::MutablePtr ColumnLowCardinality::cutAndCompact(size_t start, size_t length) const
{
    auto sub_positions = IColumn::mutate(idx.getPositions()->cut(start, length));
    
    
    auto column = ColumnLowCardinality::create(getDictionary().assumeMutable(), std::move(sub_positions));
    
    column->compactInplace();
    return column;
}
void ColumnLowCardinality::compactInplace()