I tried to rename a key column in a MergeTree table. It does not work, and 20.4 is very clear about the reason. 
20.5.2 is complaining about non-existing column with the old name, suggesting there is a column with the new name:
```
ALTER TABLE rename_key
    RENAME COLUMN key TO new

Received exception from server (version 20.4.6):
Code: 44. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key key column which is a part of key expression.

Received exception from server (version 20.5.2):
Code: 47. DB::Exception: Received from localhost:9000. 
DB::Exception: Missing columns: 'key' while processing query: 'key', required columns: 'key', source columns: 'value' 'new' 'date'.
```
The message hasn't changed for the partition key:
```
ALTER TABLE rename_key
    RENAME COLUMN date TO new

Received exception from server (version 20.4.6):
Code: 44. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key date column which is a part of key expression.

Received exception from server (version 20.5.2):
Code: 524. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key date column which is a part of key expression.
```
The table:
```
CREATE TABLE rename_key
(
    `date` Date,
    `key` UInt64,
    `value` String
)
ENGINE = MergeTree()
PARTITION BY date
ORDER BY key
```

if (codec)
            {
                
                
                if (data_type == nullptr)
                    codec->useInfoAboutType(column.type);
                else 
                    codec->useInfoAboutType(data_type);
                column.codec = codec;
            }
            if (comment)
                column.comment = *comment;
            if (ttl)
                column.ttl = ttl;
            if (data_type)
                column.type = data_type;
            
            
            if (default_expression || data_type)
            {
                column.default_desc.kind = default_kind;
                column.default_desc.expression = default_expression;
            }
        });
    }
    else if (type == MODIFY_ORDER_BY)
    {
        auto & sorting_key = metadata.sorting_key;
        auto & primary_key = metadata.primary_key;
        if (primary_key.definition_ast == nullptr && sorting_key.definition_ast != nullptr)