[SPARK-15658][SQL] UDT serializer should declare its data type as udt instead of udt.sqlType

## What changes were proposed in this pull request?

When we build serializer for UDT object, we should declare its data type as udt instead of udt.sqlType, or if we deserialize it again, we lose the information that it's a udt object and throw analysis exception.

## How was this patch tested?

new test in `UserDefiendTypeSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #13402 from cloud-fan/udt.