[SPARK-15677][SQL] Query with scalar sub-query in the SELECT list throws...

[SPARK-15677][SQL] Query with scalar sub-query in the SELECT list throws UnsupportedOperationException

## What changes were proposed in this pull request?
Queries with scalar sub-query in the SELECT list run against a local, in-memory relation throw
UnsupportedOperationException exception.

Problem repro:
```SQL
scala> Seq((1, 1), (2, 2)).toDF("c1", "c2").createOrReplaceTempView("t1")
scala> Seq((1, 1), (2, 2)).toDF("c1", "c2").createOrReplaceTempView("t2")
scala> sql("select (select min(c1) from t2) from t1").show()

java.lang.UnsupportedOperationException: Cannot evaluate expression: scalar-subquery#62 []
  at org.apache.spark.sql.catalyst.expressions.Unevaluable$class.eval(Expression.scala:215)
  at org.apache.spark.sql.catalyst.expressions.ScalarSubquery.eval(subquery.scala:62)
  at org.apache.spark.sql.catalyst.expressions.Alias.eval(namedExpressions.scala:142)
  at org.apache.spark.sql.catalyst.expressions.InterpretedProjection.apply(Projection.scala:45)
  at org.apache.spark.sql.catalyst.expressions.InterpretedProjection.apply(Projection.scala:29)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
  at scala.collection.immutable.List.foreach(List.scala:381)
  at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
  at scala.collection.immutable.List.map(List.scala:285)
  at org.apache.spark.sql.catalyst.optimizer.ConvertToLocalRelation$$anonfun$apply$37.applyOrElse(Optimizer.scala:1473)
```
The problem is specific to local, in memory relations. It is caused by rule ConvertToLocalRelation, which attempts to push down
a scalar-subquery expression to the local tables.

The solution prevents the rule to apply if Project references scalar subqueries.

## How was this patch tested?
Added regression tests to SubquerySuite.scala

Author: Ioana Delaney <ioanamdelaney@gmail.com>

Closes #13418 from ioana-delaney/scalarSubV2.