## What changes were proposed in this pull request?

Currently, `SQLBuilder` handles `LIMIT` by always adding `LIMIT` at the end of the generated subSQL. It makes `RuntimeException`s like the following. This PR adds a parenthesis always except `SubqueryAlias` is used together with `LIMIT`.

**Before**

``` scala
scala> sql("CREATE TABLE tbl(id INT)")
scala> sql("CREATE VIEW v1(id2) AS SELECT id FROM tbl LIMIT 2")
java.lang.RuntimeException: Failed to analyze the canonicalized SQL: ...
```

**After**

``` scala
scala> sql("CREATE TABLE tbl(id INT)")
scala> sql("CREATE VIEW v1(id2) AS SELECT id FROM tbl LIMIT 2")
scala> sql("SELECT id2 FROM v1")
res4: org.apache.spark.sql.DataFrame = [id2: int]
```

**Fixed cases in this PR**

The following two cases are the detail query plans having problematic SQL generations.

1. `SELECT * FROM (SELECT id FROM tbl LIMIT 2)`

    Please note that **FROM SELECT** part of the generated SQL in the below. When we don't use '()' for limit, this fails.

```scala
# Original logical plan:
Project [id#1]
+- GlobalLimit 2
   +- LocalLimit 2
      +- Project [id#1]
         +- MetastoreRelation default, tbl

# Canonicalized logical plan:
Project [gen_attr_0#1 AS id#4]
+- SubqueryAlias tbl
   +- Project [gen_attr_0#1]
      +- GlobalLimit 2
         +- LocalLimit 2
            +- Project [gen_attr_0#1]
               +- SubqueryAlias gen_subquery_0
                  +- Project [id#1 AS gen_attr_0#1]
                     +- SQLTable default, tbl, [id#1]

# Generated SQL:
SELECT `gen_attr_0` AS `id` FROM (SELECT `gen_attr_0` FROM SELECT `gen_attr_0` FROM (SELECT `id` AS `gen_attr_0` FROM `default`.`tbl`) AS gen_subquery_0 LIMIT 2) AS tbl
```

2. `SELECT * FROM (SELECT id FROM tbl TABLESAMPLE (2 ROWS))`

    Please note that **((~~~) AS gen_subquery_0 LIMIT 2)** in the below. When we use '()' for limit on `SubqueryAlias`, this fails.

```scala
# Original logical plan:
Project [id#1]
+- Project [id#1]
   +- GlobalLimit 2
      +- LocalLimit 2
         +- MetastoreRelation default, tbl

# Canonicalized logical plan:
Project [gen_attr_0#1 AS id#4]
+- SubqueryAlias tbl
   +- Project [gen_attr_0#1]
      +- GlobalLimit 2
         +- LocalLimit 2
            +- SubqueryAlias gen_subquery_0
               +- Project [id#1 AS gen_attr_0#1]
                  +- SQLTable default, tbl, [id#1]

# Generated SQL:
SELECT `gen_attr_0` AS `id` FROM (SELECT `gen_attr_0` FROM ((SELECT `id` AS `gen_attr_0` FROM `default`.`tbl`) AS gen_subquery_0 LIMIT 2)) AS tbl
```

## How was this patch tested?

Pass the Jenkins test with a newly added test case.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15546 from dongjoon-hyun/SPARK-17982.

## What changes were proposed in this pull request?

Currently, the generated SQLs have not-stable IDs for generated attributes.
The stable generated SQL will give more benefit for understanding or testing the queries.
This PR provides stable SQL generation by the followings.

 - Provide unique ids for generated subqueries, `gen_subquery_xxx`.
 - Provide unique and stable ids for generated attributes, `gen_attr_xxx`.

**Before**
```scala
scala> new org.apache.spark.sql.catalyst.SQLBuilder(sql("select 1")).toSQL
res0: String = SELECT `gen_attr_0` AS `1` FROM (SELECT 1 AS `gen_attr_0`) AS gen_subquery_0
scala> new org.apache.spark.sql.catalyst.SQLBuilder(sql("select 1")).toSQL
res1: String = SELECT `gen_attr_4` AS `1` FROM (SELECT 1 AS `gen_attr_4`) AS gen_subquery_0
```

**After**
```scala
scala> new org.apache.spark.sql.catalyst.SQLBuilder(sql("select 1")).toSQL
res1: String = SELECT `gen_attr_0` AS `1` FROM (SELECT 1 AS `gen_attr_0`) AS gen_subquery_0
scala> new org.apache.spark.sql.catalyst.SQLBuilder(sql("select 1")).toSQL
res2: String = SELECT `gen_attr_0` AS `1` FROM (SELECT 1 AS `gen_attr_0`) AS gen_subquery_0
```

## How was this patch tested?

Pass the existing Jenkins tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14257 from dongjoon-hyun/SPARK-16621.

## What changes were proposed in this pull request?

This PR improves `LogicalPlanToSQLSuite` to check the generated SQL directly by **structure**. So far, `LogicalPlanToSQLSuite` relies on  `checkHiveQl` to ensure the **successful SQL generation** and **answer equality**. However, it does not guarantee the generated SQL is the same or will not be changed unnoticeably.

## How was this patch tested?

Pass the Jenkins. This is only a testsuite change.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14235 from dongjoon-hyun/SPARK-16590.