The `SET_VAR` hint will be restored to the wrong result

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
create table ta(a int);
create binding for select count(*) from ta using select /*+ set_var(tmp_table_size=1024)*/ count(*) from ta;
```

### 2. What did you expect to see? (Required)
The binding can be created successful.

### 3. What did you see instead (Required)
```
mysql> create binding for select count(*) from ta using select /*+ set_var(tmp_table_size=1024)*/ count(*) from ta;
ERROR 1064 (42000): Optimizer hint syntax error at line 1 column 35 near "'tmp_table_size', '1024')*/"
```

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