**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. 



#include <Columns/ColumnLowCardinality.h>
#include <Columns/ColumnsNumber.h>
#include <DataStreams/ColumnGathererStream.h>
#include <DataTypes/NumberTraits.h>
#include <Common/HashTable/HashMap.h>
#include <Common/assert_cast.h>
#include <Common/WeakHash.h>
namespace DB
{
namespace ErrorCodes
{
    extern const int ILLEGAL_COLUMN;
    extern const int LOGICAL_ERROR;
}
namespace
{
    void checkColumn(const IColumn & column)
    {
        if (!dynamic_cast<const IColumnUnique *>(&column))
            throw Exception("ColumnUnique expected as an argument of ColumnLowCardinality.", ErrorCodes::ILLEGAL_COLUMN);
    }
    template <typename T>
    PaddedPODArray<T> * getIndexesData(IColumn & indexes)