[SPARK-8283][SQL] Resolve udf_struct test failure in HiveCompatibilitySuite

This PR aimed to resolve udf_struct test failure in HiveCompatibilitySuite.

Currently, this is done by loosening CreateStruct's children type from NamedExpression to Expression and automatically generating StructField name for non-NamedExpression children.

The naming convention for unnamed children follows the udf's counterpart in Hive:
`col1, col2, col3, ...`

Author: Yijie Shen <henry.yijieshen@gmail.com>

Closes #6828 from yijieshen/SPARK-8283 and squashes the following commits:

6052b73 [Yijie Shen] Doc fix
677e0b7 [Yijie Shen] Resolve udf_struct test failure by automatically generate structField name for non-NamedExpression children