[SPARK-27411][SQL] DataSourceV2Strategy should not eliminate subquery

## What changes were proposed in this pull request?

In DataSourceV2Strategy, it seems we eliminate the subqueries by mistake after normalizing filters.
We have a sql with a scalar subquery:

``` scala
val plan = spark.sql("select * from t2 where t2a > (select max(t1a) from t1)")
plan.explain(true)
```

And we get the log info of DataSourceV2Strategy:
```
Pushing operators to csv:examples/src/main/resources/t2.txt
Pushed Filters:
Post-Scan Filters: isnotnull(t2a#30)
Output: t2a#30, t2b#31
```

The `Post-Scan Filters` should contain the scalar subquery, but we eliminate it by mistake.
```
== Parsed Logical Plan ==
'Project [*]
+- 'Filter ('t2a > scalar-subquery#56 [])
   :  +- 'Project [unresolvedalias('max('t1a), None)]
   :     +- 'UnresolvedRelation `t1`
   +- 'UnresolvedRelation `t2`

== Analyzed Logical Plan ==
t2a: string, t2b: string
Project [t2a#30, t2b#31]
+- Filter (t2a#30 > scalar-subquery#56 [])
   :  +- Aggregate [max(t1a#13) AS max(t1a)apache-spark-on-k8s#63]
   :     +- SubqueryAlias `t1`
   :        +- RelationV2[t1a#13, t1b#14] csv:examples/src/main/resources/t1.txt
   +- SubqueryAlias `t2`
      +- RelationV2[t2a#30, t2b#31] csv:examples/src/main/resources/t2.txt

== Optimized Logical Plan ==
Filter (isnotnull(t2a#30) && (t2a#30 > scalar-subquery#56 []))
:  +- Aggregate [max(t1a#13) AS max(t1a)apache-spark-on-k8s#63]
:     +- Project [t1a#13]
:        +- RelationV2[t1a#13, t1b#14] csv:examples/src/main/resources/t1.txt
+- RelationV2[t2a#30, t2b#31] csv:examples/src/main/resources/t2.txt

== Physical Plan ==
*(1) Project [t2a#30, t2b#31]
+- *(1) Filter isnotnull(t2a#30)
   +- *(1) BatchScan[t2a#30, t2b#31] class org.apache.spark.sql.execution.datasources.v2.csv.CSVScan
```
## How was this patch tested?

ut

Closes apache#24321 from francis0407/SPARK-27411.

Authored-by: francis0407 <hanmingcong123@hotmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DataSourceV2Strategy.scala

@@ -106,13 +106,16 @@ object DataSourceV2Strategy extends Strategy with PredicateHelper {
     case PhysicalOperation(project, filters, relation: DataSourceV2Relation) =>
       val scanBuilder = relation.newScanBuilder()
 
+      val (withSubquery, withoutSubquery) = filters.partition(SubqueryExpression.hasSubquery)
       val normalizedFilters = DataSourceStrategy.normalizeFilters(
-        filters.filterNot(SubqueryExpression.hasSubquery), relation.output)
+        withoutSubquery, relation.output)
 
       // `pushedFilters` will be pushed down and evaluated in the underlying data sources.
       // `postScanFilters` need to be evaluated after the scan.
       // `postScanFilters` and `pushedFilters` can overlap, e.g. the parquet row group filter.
-      val (pushedFilters, postScanFilters) = pushFilters(scanBuilder, normalizedFilters)
+      val (pushedFilters, postScanFiltersWithoutSubquery) =
+        pushFilters(scanBuilder, normalizedFilters)
+      val postScanFilters = postScanFiltersWithoutSubquery ++ withSubquery
       val (scan, output) = pruneColumns(scanBuilder, relation, project ++ postScanFilters)
       logInfo(
         s"""

sql/core/src/test/scala/org/apache/spark/sql/sources/v2/DataSourceV2Suite.scala

@@ -392,6 +392,19 @@ class DataSourceV2Suite extends QueryTest with SharedSQLContext {
       }
     }
   }
+
+  test("SPARK-27411: DataSourceV2Strategy should not eliminate subquery") {
+    withTempView("t1") {
+      val t2 = spark.read.format(classOf[SimpleDataSourceV2].getName).load()
+      Seq(2, 3).toDF("a").createTempView("t1")
+      val df = t2.where("i < (select max(a) from t1)").select('i)
+      val subqueries = df.queryExecution.executedPlan.collect {
+        case p => p.subqueries
+      }.flatten
+      assert(subqueries.length == 1)
+      checkAnswer(df, (0 until 3).map(i => Row(i)))
+    }
+  }
 }