Cast from utf8mb4 to latin1 is not compatible with MySQL

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

Run SQL `SELECT CAST('中文' AS CHAR CHARACTER SET latin1);`

### 2. What did you expect to see? (Required)

```
mysql> SELECT CAST('中文' AS CHAR CHARACTER SET latin1);
+---------------------------------------------+
| CAST('中文' AS CHAR CHARACTER SET latin1) |
+---------------------------------------------+
| ?? |
+---------------------------------------------+
1 row in set (0.01 sec)
```

### 3. What did you see instead (Required)

```
mysql> SELECT CAST('中文' AS CHAR CHARACTER SET latin1);
+---------------------------------------------+
| CAST('中文' AS CHAR CHARACTER SET latin1) |
+---------------------------------------------+
| 中文 |
+---------------------------------------------+
1 row in set (0.00 sec)
```

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