[SPARK-3300][SQL] No need to call clear() and shorten build()

The function `ensureFreeSpace` in object `ColumnBuilder` clears old buffer before copying its content to new buffer. This PR fixes it.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #2195 from viirya/fix_buffer_clear and squashes the following commits:

792f009 [Liang-Chi Hsieh] no need to call clear(). use flip() instead of calling limit(), position() and rewind().
df2169f [Liang-Chi Hsieh] should clean old buffer after copying its content.

sql/core/src/main/scala/org/apache/spark/sql/columnar/ColumnBuilder.scala

@@ -75,8 +75,7 @@ private[sql] class BasicColumnBuilder[T <: DataType, JvmType](
   }
 
   override def build() = {
-    buffer.limit(buffer.position()).rewind()
-    buffer
+    buffer.flip().asInstanceOf[ByteBuffer]
   }
 }
 
@@ -129,7 +128,6 @@ private[sql] object ColumnBuilder {
       val newSize = capacity + size.max(capacity / 8 + 1)
       val pos = orig.position()
 
-      orig.clear()
       ByteBuffer
         .allocate(newSize)
         .order(ByteOrder.nativeOrder())