[SPARK-18368] Fix regexp_replace with task serialization.

## What changes were proposed in this pull request?

This makes the result value both transient and lazy, so that if the RegExpReplace object is initialized then serialized, `result: StringBuffer` will be correctly initialized.

## How was this patch tested?

* Verified that this patch fixed the query that found the bug.
* Added a test case that fails without the fix.

Author: Ryan Blue <blue@apache.org>

Closes #15816 from rdblue/SPARK-18368-fix-regexp-replace.

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

@@ -230,7 +230,7 @@ case class RegExpReplace(subject: Expression, regexp: Expression, rep: Expressio
   @transient private var lastReplacement: String = _
   @transient private var lastReplacementInUTF8: UTF8String = _
   // result buffer write by Matcher
-  @transient private val result: StringBuffer = new StringBuffer
+  @transient private lazy val result: StringBuffer = new StringBuffer
 
   override def nullSafeEval(s: Any, p: Any, r: Any): Any = {
     if (!p.equals(lastRegex)) {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvalHelper.scala

@@ -22,7 +22,8 @@ import org.scalactic.TripleEqualsSupport.Spread
 import org.scalatest.exceptions.TestFailedException
 import org.scalatest.prop.GeneratorDrivenPropertyChecks
 
-import org.apache.spark.SparkFunSuite
+import org.apache.spark.{SparkConf, SparkFunSuite}
+import org.apache.spark.serializer.JavaSerializer
 import org.apache.spark.sql.catalyst.{CatalystTypeConverters, InternalRow}
 import org.apache.spark.sql.catalyst.expressions.codegen._
 import org.apache.spark.sql.catalyst.optimizer.SimpleTestOptimizer
@@ -43,13 +44,15 @@ trait ExpressionEvalHelper extends GeneratorDrivenPropertyChecks {
 
   protected def checkEvaluation(
       expression: => Expression, expected: Any, inputRow: InternalRow = EmptyRow): Unit = {
+    val serializer = new JavaSerializer(new SparkConf()).newInstance
+    val expr: Expression = serializer.deserialize(serializer.serialize(expression))
     val catalystValue = CatalystTypeConverters.convertToCatalyst(expected)
-    checkEvaluationWithoutCodegen(expression, catalystValue, inputRow)
-    checkEvaluationWithGeneratedMutableProjection(expression, catalystValue, inputRow)
-    if (GenerateUnsafeProjection.canSupport(expression.dataType)) {
-      checkEvalutionWithUnsafeProjection(expression, catalystValue, inputRow)
+    checkEvaluationWithoutCodegen(expr, catalystValue, inputRow)
+    checkEvaluationWithGeneratedMutableProjection(expr, catalystValue, inputRow)
+    if (GenerateUnsafeProjection.canSupport(expr.dataType)) {
+      checkEvalutionWithUnsafeProjection(expr, catalystValue, inputRow)
     }
-    checkEvaluationWithOptimization(expression, catalystValue, inputRow)
+    checkEvaluationWithOptimization(expr, catalystValue, inputRow)
   }
 
   /**