**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. 



: column_unique(std::move(column_unique_)), shared(is_shared)
{
    checkColumn(*column_unique);
}
ColumnLowCardinality::Dictionary::Dictionary(ColumnPtr column_unique_, bool is_shared)
    : column_unique(std::move(column_unique_)), shared(is_shared)
{
    checkColumn(*column_unique);
}
void ColumnLowCardinality::Dictionary::setShared(const ColumnPtr & column_unique_)
{
    checkColumn(*column_unique_);
    column_unique = column_unique_;
    shared = true;
}
void ColumnLowCardinality::Dictionary::compact(ColumnPtr & positions)
{
    auto new_column_unique = column_unique->cloneEmpty();
    auto & unique = getColumnUnique();
    auto & new_unique = static_cast<IColumnUnique &>(*new_column_unique);
    auto indexes = mapUniqueIndex(positions->assumeMutableRef());
    auto sub_keys = unique.getNestedColumn()->index(*indexes, 0);
    auto new_indexes = new_unique.uniqueInsertRangeFrom(*sub_keys, 0, sub_keys->size());
    positions = IColumn::mutate(new_indexes->index(*positions, 0));
    column_unique = std::move(new_column_unique);
    shared = false;
}
}