[SPARK-17368][SQL] Add support for value class serialization and deserialization

## What changes were proposed in this pull request?
Value classes were unsupported because catalyst data types were
obtained through reflection on erased types, which would resolve to a
value class' wrapped type and hence lead to unavailable methods during
code generation.

E.g. the following class
```scala
case class Foo(x: Int) extends AnyVal
```
would be seen as an `int` in catalyst and will cause instance cast failures when generated java code tries to treat it as a `Foo`.

This patch simply removes the erasure step when getting data types for
catalyst.

## How was this patch tested?
Additional tests in `ExpressionEncoderSuite`.

Author: Jakob Odersky <jakob@odersky.com>

Closes #15284 from jodersky/value-classes.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala

@@ -628,7 +628,7 @@ object ScalaReflection extends ScalaReflection {
   /*
    * Retrieves the runtime class corresponding to the provided type.
    */
-  def getClassFromType(tpe: Type): Class[_] = mirror.runtimeClass(tpe.erasure.typeSymbol.asClass)
+  def getClassFromType(tpe: Type): Class[_] = mirror.runtimeClass(tpe.typeSymbol.asClass)
 
   case class Schema(dataType: DataType, nullable: Boolean)
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoderSuite.scala

@@ -66,8 +66,6 @@ case class RepeatedData(
     mapFieldNull: scala.collection.Map[Int, java.lang.Long],
     structField: PrimitiveData)
 
-case class SpecificCollection(l: List[Int])
-
 /** For testing Kryo serialization based encoder. */
 class KryoSerializable(val value: Int) {
   override def hashCode(): Int = value
@@ -107,6 +105,12 @@ class UDTForCaseClass extends UserDefinedType[UDTCaseClass] {
   }
 }
 
+case class PrimitiveValueClass(wrapped: Int) extends AnyVal
+case class ReferenceValueClass(wrapped: ReferenceValueClass.Container) extends AnyVal
+object ReferenceValueClass {
+  case class Container(data: Int)
+}
+
 class ExpressionEncoderSuite extends PlanTest with AnalysisTest {
   OuterScopes.addOuterScope(this)
 
@@ -290,6 +294,12 @@ class ExpressionEncoderSuite extends PlanTest with AnalysisTest {
     ExpressionEncoder.tuple(intEnc, ExpressionEncoder.tuple(intEnc, longEnc))
   }
 
+  encodeDecodeTest(
+    PrimitiveValueClass(42), "primitive value class")
+
+  encodeDecodeTest(
+    ReferenceValueClass(ReferenceValueClass.Container(1)), "reference value class")
+
   productTest(("UDT", new ExamplePoint(0.1, 0.2)))
 
   test("nullable of encoder schema") {