aggregate function calls cannot be nested, should result error

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```sql
drop table if exists t1,t2;
CREATE TABLE t1 (a INT); INSERT INTO t1 values (1),(1),(1),(1);
CREATE TABLE t2 (x INT); INSERT INTO t1 values (1000),(1001),(1002);
SELECT SUM( (SELECT COUNT(a) FROM t2) ) FROM t1;
```
### 2. What did you expect to see? (Required)
```sql
mysql> SELECT SUM( (SELECT COUNT(a) FROM t2) ) FROM t1;
ERROR 1111 (HY000): Invalid use of group function
```
### 3. What did you see instead (Required)
```sql
mysql> SELECT SUM( (SELECT COUNT(a) FROM t2) ) FROM t1;
+----------------------------------+
| SUM( (SELECT COUNT(a) FROM t2) ) |
+----------------------------------+
| 0 |
+----------------------------------+
1 row in set (0.00 sec)
```
### 4. What is your TiDB version? (Required)

```sql
mysql> select version();
+-----------------------------------------------+
| version() |
+-----------------------------------------------+
| 5.7.25-TiDB-v6.1.0-alpha-482-g81a852077-dirty |
+-----------------------------------------------+
```