(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

{
    auto set_key = PreparedSetKey::forSubquery(*subquery_or_table_name);
    if (prepared_sets.count(set_key))
        return; 
    if (auto set_ptr_from_storage_set = isPlainStorageSetInSubquery(subquery_or_table_name))
    {
        prepared_sets.insert({set_key, set_ptr_from_storage_set});
        return;
    }
    auto interpreter_subquery = interpretSubquery(subquery_or_table_name, getContext(), {}, query_options);
    auto io = interpreter_subquery->execute();
    PullingAsyncPipelineExecutor executor(io.pipeline);
    SetPtr set = std::make_shared<Set>(settings.size_limits_for_set, true, getContext()->getSettingsRef().transform_null_in);
    set->setHeader(executor.getHeader());
    Block block;
    while (executor.pull(block))
    {
        if (block.rows() == 0)
            continue;
        
        if (!set->insertFromBlock(block))
            return;
    }
    set->finishInsert();
    prepared_sets[set_key] = std::move(set);
}
SetPtr ExpressionAnalyzer::isPlainStorageSetInSubquery(const ASTPtr & subquery_or_table_name)