[MINOR] Resolve a number of miscellaneous build warnings

## What changes were proposed in this pull request?

This change resolves a number of build warnings that have accumulated, before 2.x. It does not address a large number of deprecation warnings, especially related to the Accumulator API. That will happen separately.

## How was this patch tested?

Jenkins

Author: Sean Owen <sowen@cloudera.com>

Closes #13377 from srowen/BuildWarnings.

common/unsafe/src/main/java/org/apache/spark/unsafe/Platform.java

@@ -155,8 +155,8 @@ public final class Platform {
   @SuppressWarnings("unchecked")
   public static ByteBuffer allocateDirectBuffer(int size) {
     try {
-      Class cls = Class.forName("java.nio.DirectByteBuffer");
+      Class<?> cls = Class.forName("java.nio.DirectByteBuffer");
-      Constructor constructor = cls.getDeclaredConstructor(Long.TYPE, Integer.TYPE);
+      Constructor<?> constructor = cls.getDeclaredConstructor(Long.TYPE, Integer.TYPE);
       constructor.setAccessible(true);
       Field cleanerField = cls.getDeclaredField("cleaner");
       cleanerField.setAccessible(true);

mllib/src/test/java/org/apache/spark/mllib/fpm/JavaPrefixSpanSuite.java

@@ -72,7 +72,9 @@ public class JavaPrefixSpanSuite extends SharedSparkSession {
 
     try {
       model.save(spark.sparkContext(), outputPath);
-      PrefixSpanModel newModel = PrefixSpanModel.load(spark.sparkContext(), outputPath);
+      @SuppressWarnings("unchecked")
+      PrefixSpanModel<Integer> newModel =
+          (PrefixSpanModel<Integer>) PrefixSpanModel.load(spark.sparkContext(), outputPath);
       JavaRDD<FreqSequence<Integer>> freqSeqs = newModel.freqSequences().toJavaRDD();
       List<FreqSequence<Integer>> localFreqSeqs = freqSeqs.collect();
       Assert.assertEquals(5, localFreqSeqs.size());

mllib/src/test/scala/org/apache/spark/mllib/fpm/PrefixSpanSuite.scala

@@ -16,6 +16,8 @@
  */
 package org.apache.spark.mllib.fpm
 
+import scala.language.existentials
+
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.mllib.util.MLlibTestSparkContext
 import org.apache.spark.util.Utils

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/ScalaReflectionSuite.scala

@@ -288,7 +288,7 @@ class ScalaReflectionSuite extends SparkFunSuite {
     assert(serializer.children.head.asInstanceOf[Literal].value === UTF8String.fromString("value"))
     assert(serializer.children.last.isInstanceOf[NewInstance])
     assert(serializer.children.last.asInstanceOf[NewInstance]
-      .cls.isInstanceOf[Class[org.apache.spark.sql.catalyst.util.GenericArrayData]])
+      .cls.isAssignableFrom(classOf[org.apache.spark.sql.catalyst.util.GenericArrayData]))
   }
 
   private val dataTypeForComplexData = dataTypeFor[ComplexData]

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

@@ -365,7 +365,8 @@ class ExpressionEncoderSuite extends PlanTest with AnalysisTest {
           Arrays.deepEquals(b1.asInstanceOf[Array[AnyRef]], b2.asInstanceOf[Array[AnyRef]])
         case (b1: Array[_], b2: Array[_]) =>
           Arrays.equals(b1.asInstanceOf[Array[AnyRef]], b2.asInstanceOf[Array[AnyRef]])
-        case (left: Comparable[Any], right: Comparable[Any]) => left.compareTo(right) == 0
+        case (left: Comparable[_], right: Comparable[_]) =>
+          left.asInstanceOf[Comparable[Any]].compareTo(right) == 0
         case _ => input == convertedBack
       }
 

sql/core/src/test/scala/org/apache/spark/sql/JoinSuite.scala

@@ -17,6 +17,8 @@
 
 package org.apache.spark.sql
 
+import scala.language.existentials
+
 import org.apache.spark.sql.catalyst.analysis.UnresolvedRelation
 import org.apache.spark.sql.catalyst.TableIdentifier
 import org.apache.spark.sql.execution.joins._

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/FileCatalogSuite.scala

@@ -19,6 +19,8 @@ package org.apache.spark.sql.execution.datasources
 
 import java.io.File
 
+import scala.language.reflectiveCalls
+
 import org.apache.hadoop.fs.Path
 
 import org.apache.spark.sql.catalyst.util._

yarn/src/test/scala/org/apache/spark/network/yarn/YarnShuffleServiceSuite.scala

@@ -20,6 +20,7 @@ import java.io.{DataOutputStream, File, FileOutputStream}
 
 import scala.annotation.tailrec
 import scala.concurrent.duration._
+import scala.language.postfixOps
 
 import org.apache.hadoop.fs.Path
 import org.apache.hadoop.yarn.api.records.ApplicationId
@@ -27,7 +28,6 @@ import org.apache.hadoop.yarn.conf.YarnConfiguration
 import org.apache.hadoop.yarn.server.api.{ApplicationInitializationContext, ApplicationTerminationContext}
 import org.scalatest.{BeforeAndAfterEach, Matchers}
 import org.scalatest.concurrent.Eventually._
-import org.scalatest.concurrent.Timeouts
 
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.network.shuffle.ShuffleTestAccessor