[SPARK-21129][SQL] Arguments of SQL function call should not be named expressions

### What changes were proposed in this pull request?

Function argument should not be named expressions. It could cause two issues:
- Misleading error message
- Unexpected query results when the column name is `distinct`, which is not a reserved word in our parser.

```
spark-sql> select count(distinct c1, distinct c2) from t1;
Error in query: cannot resolve '`distinct`' given input columns: [c1, c2]; line 1 pos 26;
'Project [unresolvedalias('count(c1#30, 'distinct), None)]
+- SubqueryAlias t1
   +- CatalogRelation `default`.`t1`, org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe, [c1#30, c2#31]
```

After the fix, the error message becomes
```
spark-sql> select count(distinct c1, distinct c2) from t1;
Error in query:
extraneous input 'c2' expecting {')', ',', '.', '[', 'OR', 'AND', 'IN', NOT, 'BETWEEN', 'LIKE', RLIKE, 'IS', EQ, '<=>', '<>', '!=', '<', LTE, '>', GTE, '+', '-', '*', '/', '%', 'DIV', '&', '|', '||', '^'}(line 1, pos 35)

== SQL ==
select count(distinct c1, distinct c2) from t1
-----------------------------------^^^
```

### How was this patch tested?
Added a test case to parser suite.

Author: Xiao Li <gatorsmile@gmail.com>
Author: gatorsmile <gatorsmile@gmail.com>

Closes #18338 from gatorsmile/parserDistinctAggFunc.