(you don't have to strictly follow this form)

**Describe the bug**
When using `ORDER BY column_name COLLATE 'en'`, `COLLATE` is ignored if `column_name` is the primary key
 
**How to reproduce**
* version 21.1.2
* `CREATE TABLE` statements for all tables involved:
```
CREATE TABLE test.test_collation
(
    `v` String,
    `v2` String
)
ENGINE = MergeTree
ORDER BY v
SETTINGS index_granularity = 8192
```
* Sample data:
```
insert into test.test_collation values ('A', 'A')
insert into test.test_collation values ('B', 'B')
insert into test.test_collation values ('C', 'C')
insert into test.test_collation values ('a', 'a')
insert into test.test_collation values ('b', 'b')
insert into test.test_collation values ('c', 'c')
```
* Queries to run that lead to unexpected result
Primary key
```
SELECT *
FROM test.test_collation
ORDER BY v ASC COLLATE 'en'

Query id: fa84ae7a-abe7-4886-99cd-eba7ad5d5fc4

┌─v─┬─v2─┐
│ A │ A  │
│ B │ B  │
│ C │ C  │
│ a │ a  │
│ b │ b  │
│ c │ c  │
└───┴────┘
```
Separate column
```
SELECT *
FROM test.test_collation
ORDER BY v2 ASC COLLATE 'en'

Query id: 04401f85-3cb2-442c-8c89-e3328d1615cd

┌─v─┬─v2─┐
│ a │ a  │
│ A │ A  │
│ b │ b  │
│ B │ B  │
│ c │ c  │
│ C │ C  │
└───┴────┘
```

**Expected behavior**
Both of these queries to return the rows in the same order

{
        ss << "array_join " << "FIXME doesn't have dump" << "\n";
    }
    if (before_join)
    {
        ss << "before_join " << before_join->dumpDAG() << "\n";
    }
    if (before_where)
    {
        ss << "before_where " << before_where->dumpDAG() << "\n";
    }
    if (prewhere_info)
    {
        ss << "prewhere_info " << prewhere_info->dump() << "\n";
    }
    if (filter_info)
    {
        ss << "filter_info " << filter_info->dump() << "\n";
    }
    if (before_aggregation)
    {
        ss << "before_aggregation " << before_aggregation->dumpDAG() << "\n";
    }
    if (before_having)
    {
        ss << "before_having " << before_having->dumpDAG() << "\n";
    }
    if (before_window)
    {
        ss << "before_window " << before_window->dumpDAG() << "\n";