Planner: can't create a binding for TTL SQL

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
create binding for ttl query
```
mysql> CREATE GLOBAL BINDING FOR SELECT LOW_PRIORITY SQL_NO_CACHE `id` FROM `test`.`t1` WHERE `created_at` < FROM_UNIXTIME(?) ORDER BY `id` ASC LIMIT ? USING SELECT /*+ RESOURCE_GROUP(rg1) */ LOW_PRIORITY SQL_NO_CACHE `id` FROM `test`.`t1` WHERE `created_at` < FROM_UNIXTIME(?) ORDER BY `id` ASC LIMIT ?;
```

### 2. What did you expect to see? (Required)
create success
### 3. What did you see instead (Required)
```
mysql> CREATE GLOBAL BINDING FOR SELECT LOW_PRIORITY SQL_NO_CACHE `id` FROM `test`.`t1` WHERE `created_at` < FROM_UNIXTIME(?) ORDER BY `id` ASC LIMIT ? USING SELECT /*+ RESOURCE_GROUP(rg1) */ LOW_PRIORITY SQL_NO_CACHE `id` FROM `test`.`t1` WHERE `created_at` < FROM_UNIXTIME(?) ORDER BY `id` ASC LIMIT ?;
ERROR 8066 (HY000): Optimizer hint can only be followed by certain keywords like SELECT, INSERT, etc.
```

```
### 4. What is your TiDB version? (Required)

No source file or test is named. Start by reproducing the CREATE GLOBAL BINDING command and trace the planner's binding path for the RESOURCE_GROUP optimizer hint; done means the command succeeds and the regression is covered by a test.

Written by the indexing model from the issue text.