[SPARK-24505][SQL] Convert strings in codegen to blocks: Cast and BoundAttribute

## What changes were proposed in this pull request?

This is split from #21520. This includes changes of `BoundAttribute` and `Cast`.
This patch also adds few convenient APIs:

```scala
CodeGenerator.freshVariable(name: String, dt: DataType): VariableValue
CodeGenerator.freshVariable(name: String, javaClass: Class[_]): VariableValue

JavaCode.javaType(javaClass: Class[_]): Inline
JavaCode.javaType(dataType: DataType): Inline
JavaCode.boxedType(dataType: DataType): Inline
```

## How was this patch tested?

Existing tests.

Closes #21537 from viirya/SPARK-24505-1.

Authored-by: Liang-Chi Hsieh <[email protected]>
Signed-off-by: hyukjinkwon <[email protected]>

Author: viirya

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

@@ -20,7 +20,7 @@ package org.apache.spark.sql.catalyst.expressions
 import org.apache.spark.internal.Logging
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.errors.attachTree
-import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, CodeGenerator, ExprCode, FalseLiteral}
+import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, CodeGenerator, ExprCode, FalseLiteral, JavaCode}
 import org.apache.spark.sql.catalyst.expressions.codegen.Block._
 import org.apache.spark.sql.types._
 
@@ -53,7 +53,7 @@ case class BoundReference(ordinal: Int, dataType: DataType, nullable: Boolean)
       ev.copy(code = oev.code)
     } else {
       assert(ctx.INPUT_ROW != null, "INPUT_ROW and currentVars cannot both be null.")
-      val javaType = CodeGenerator.javaType(dataType)
+      val javaType = JavaCode.javaType(dataType)
       val value = CodeGenerator.getValue(ctx.INPUT_ROW, dataType, ordinal.toString)
       if (nullable) {
         ev.copy(code =