[SPARK-17741][SQL] Grammar to parse top level and nested data fields separately

## What changes were proposed in this pull request?

Currently we use the same rule to parse top level and nested data fields. For example:
```
create table tbl_x(
  id bigint,
  nested struct<col1:string,col2:string>
)
```
Shows both syntaxes. In this PR we split this rule in a top-level and nested rule.

Before this PR,
```
sql("CREATE TABLE my_tab(column1: INT)")
```
works fine.
After this PR, it will throw a `ParseException`:
```
scala> sql("CREATE TABLE my_tab(column1: INT)")
org.apache.spark.sql.catalyst.parser.ParseException:
no viable alternative at input 'CREATE TABLE my_tab(column1:'(line 1, pos 27)
```

## How was this patch tested?
Add new testcases in `SparkSqlParserSuite`.

Author: jiangxingbo <jiangxb1987@gmail.com>

Closes #15346 from jiangxb1987/cdt.