[SPARK-21203][SQL] Fix wrong results of insertion of Array of Struct

### What changes were proposed in this pull request?
```SQL
CREATE TABLE `tab1`
(`custom_fields` ARRAY<STRUCT<`id`: BIGINT, `value`: STRING>>)
USING parquet

INSERT INTO `tab1`
SELECT ARRAY(named_struct('id', 1, 'value', 'a'), named_struct('id', 2, 'value', 'b'))

SELECT custom_fields.id, custom_fields.value FROM tab1
```

The above query always return the last struct of the array, because the rule `SimplifyCasts` incorrectly rewrites the query. The underlying cause is we always use the same `GenericInternalRow` object when doing the cast.

### How was this patch tested?

Author: gatorsmile <gatorsmile@gmail.com>

Closes #18412 from gatorsmile/castStruct.