**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. 



{
    return getDictionary().serializeValueIntoArena(getIndexes().getUInt(n), arena, begin);
}
const char * ColumnLowCardinality::deserializeAndInsertFromArena(const char * pos)
{
    compactIfSharedDictionary();
    const char * new_pos;
    idx.insertPosition(dictionary.getColumnUnique().uniqueDeserializeAndInsertFromArena(pos, new_pos));
    idx.check(getDictionary().size());
    return new_pos;
}
void ColumnLowCardinality::updateWeakHash32(WeakHash32 & hash) const
{
    auto s = size();
    if (hash.getData().size() != s)
        throw Exception("Size of WeakHash32 does not match size of column: column size is " + std::to_string(s) +
                        ", hash size is " + std::to_string(hash.getData().size()), ErrorCodes::LOGICAL_ERROR);
    const auto & dict = getDictionary().getNestedColumn();
    WeakHash32 dict_hash(dict->size());
    dict->updateWeakHash32(dict_hash);
    idx.updateWeakHash(hash, dict_hash);
}
void ColumnLowCardinality::updateHashFast(SipHash & hash) const
{
    idx.getPositions()->updateHashFast(hash);
    getDictionary().getNestedColumn()->updateHashFast(hash);
}
void ColumnLowCardinality::gather(ColumnGathererStream & gatherer)