**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 positions = idx.detachPositions();
    dictionary.compact(positions);
    idx.attachPositions(std::move(positions));
}
void ColumnLowCardinality::compactIfSharedDictionary()
{
    if (dictionary.isShared())
        compactInplace();
}
ColumnLowCardinality::DictionaryEncodedColumn
ColumnLowCardinality::getMinimalDictionaryEncodedColumn(UInt64 offset, UInt64 limit) const
{
    MutableColumnPtr sub_indexes = IColumn::mutate(idx.getPositions()->cut(offset, limit));
    auto indexes_map = mapUniqueIndex(*sub_indexes);
    auto sub_keys = getDictionary().getNestedColumn()->index(*indexes_map, 0);
    return {std::move(sub_keys), std::move(sub_indexes)};
}
ColumnPtr ColumnLowCardinality::countKeys() const
{
    const auto & nested_column = getDictionary().getNestedColumn();
    size_t dict_size = nested_column->size();
    auto counter = ColumnUInt64::create(dict_size, 0);
    idx.countKeys(counter->getData());
    return counter;
}
bool ColumnLowCardinality::containsNull() const