[SPARK-17703][SQL] Add unnamed version of addReferenceObj for minor objects.

## What changes were proposed in this pull request?

There are many minor objects in references, which are extracted to the generated class field, e.g. `errMsg` in `GetExternalRowField` or `ValidateExternalType`, but number of fields in class is limited so we should reduce the number.
This pr adds unnamed version of `addReferenceObj` for these minor objects not to store the object into field but refer it from the `references` field at the time of use.

## How was this patch tested?

Existing tests.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #15276 from ueshin/issues/SPARK-17703.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -84,6 +84,21 @@ class CodegenContext {
    */
   val references: mutable.ArrayBuffer[Any] = new mutable.ArrayBuffer[Any]()
 
+  /**
+   * Add an object to `references`.
+   *
+   * Returns the code to access it.
+   *
+   * This is for minor objects not to store the object into field but refer it from the references
+   * field at the time of use because number of fields in class is limited so we should reduce it.
+   */
+  def addReferenceObj(obj: Any): String = {
+    val idx = references.length
+    references += obj
+    val clsName = obj.getClass.getName
+    s"(($clsName) references[$idx])"
+  }
+
   /**
    * Add an object to `references`, create a class member to access it.
    *

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/misc.scala

@@ -517,7 +517,10 @@ case class AssertTrue(child: Expression) extends UnaryExpression with ImplicitCa
 
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val eval = child.genCode(ctx)
-    val errMsgField = ctx.addReferenceObj("errMsg", errMsg)
+
+    // Use unnamed reference that doesn't create a local field here to reduce the number of fields
+    // because errMsgField is used only when the value is null or false.
+    val errMsgField = ctx.addReferenceObj(errMsg)
     ExprCode(code = s"""${eval.code}
        |if (${eval.isNull} || !${eval.value}) {
        |  throw new RuntimeException($errMsgField);

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala

@@ -906,7 +906,9 @@ case class AssertNotNull(child: Expression, walkedTypePath: Seq[String])
   override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val childGen = child.genCode(ctx)
 
-    val errMsgField = ctx.addReferenceObj("errMsg", errMsg)
+    // Use unnamed reference that doesn't create a local field here to reduce the number of fields
+    // because errMsgField is used only when the value is null.
+    val errMsgField = ctx.addReferenceObj(errMsg)
 
     val code = s"""
       ${childGen.code}
@@ -941,7 +943,9 @@ case class GetExternalRowField(
   private val errMsg = s"The ${index}th field '$fieldName' of input row cannot be null."
 
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
-    val errMsgField = ctx.addReferenceObj("errMsg", errMsg)
+    // Use unnamed reference that doesn't create a local field here to reduce the number of fields
+    // because errMsgField is used only when the field is null.
+    val errMsgField = ctx.addReferenceObj(errMsg)
     val row = child.genCode(ctx)
     val code = s"""
       ${row.code}
@@ -979,7 +983,9 @@ case class ValidateExternalType(child: Expression, expected: DataType)
   private val errMsg = s" is not a valid external type for schema of ${expected.simpleString}"
 
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
-    val errMsgField = ctx.addReferenceObj("errMsg", errMsg)
+    // Use unnamed reference that doesn't create a local field here to reduce the number of fields
+    // because errMsgField is used only when the type doesn't match.
+    val errMsgField = ctx.addReferenceObj(errMsg)
     val input = child.genCode(ctx)
     val obj = input.value