[SPARK-25979][SQL] Window function: allow parentheses around window reference

## What changes were proposed in this pull request?

Very minor parser bug, but possibly problematic for code-generated queries:

Consider the following two queries:
```
SELECT avg(k) OVER (w) FROM kv WINDOW w AS (PARTITION BY v ORDER BY w) ORDER BY 1
```
and
```
SELECT avg(k) OVER w FROM kv WINDOW w AS (PARTITION BY v ORDER BY w) ORDER BY 1
```
The former, with parens around the OVER condition, fails to parse while the latter, without parens, succeeds:
```
Error in SQL statement: ParseException:
mismatched input '(' expecting {<EOF>, ',', 'FROM', 'WHERE', 'GROUP', 'ORDER', 'HAVING', 'LIMIT', 'LATERAL', 'WINDOW', 'UNION', 'EXCEPT', 'MINUS', 'INTERSECT', 'SORT', 'CLUSTER', 'DISTRIBUTE'}(line 1, pos 19)

== SQL ==
SELECT avg(k) OVER (w) FROM kv WINDOW w AS (PARTITION BY v ORDER BY w) ORDER BY 1
-------------------^^^
```
This was found when running the cockroach DB tests.

I tried PostgreSQL, The SQL with parentheses  is also workable.

## How was this patch tested?

Unit test

Closes apache#22987 from gengliangwang/windowParentheses.

Authored-by: Gengliang Wang <[email protected]>
Signed-off-by: gatorsmile <[email protected]>

(cherry picked from commit 1db7997)

Signed-off-by: gatorsmile <[email protected]>

Author: gengliangwang

sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4

@@ -690,6 +690,7 @@ namedWindow
 
 windowSpec
     : name=identifier  #windowRef
+    | '('name=identifier')'  #windowRef
     | '('
       ( CLUSTER BY partition+=expression (',' partition+=expression)*
       | ((PARTITION | DISTRIBUTE) BY partition+=expression (',' partition+=expression)*)?

sql/core/src/test/resources/sql-tests/inputs/window.sql

@@ -109,3 +109,9 @@ last_value(false, false) OVER w AS last_value_contain_null
 FROM testData
 WINDOW w AS ()
 ORDER BY cate, val;
+
+-- parentheses around window reference
+SELECT cate, sum(val) OVER (w)
+FROM testData
+WHERE val is not null
+WINDOW w AS (PARTITION BY cate ORDER BY val);

sql/core/src/test/resources/sql-tests/results/window.sql.out

@@ -1,5 +1,5 @@
 -- Automatically generated by SQLQueryTestSuite
--- Number of queries: 22
+-- Number of queries: 23
 
 
 -- !query 0
@@ -363,3 +363,20 @@ NULL	a	false	true	false	false	true	false
 1	b	false	true	false	false	true	false
 2	b	false	true	false	false	true	false
 3	b	false	true	false	false	true	false
+
+
+-- !query 22
+SELECT cate, sum(val) OVER (w)
+FROM testData
+WHERE val is not null
+WINDOW w AS (PARTITION BY cate ORDER BY val)
+-- !query 22 schema
+struct<cate:string,sum(CAST(val AS BIGINT)) OVER (PARTITION BY cate ORDER BY val ASC NULLS FIRST RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW):bigint>
+-- !query 22 output
+NULL	3
+a	2
+a	2
+a	4
+b	1
+b	3
+b	6