[SPARK-18216][SQL] Make Column.expr public

## What changes were proposed in this pull request?
Column.expr is private[sql], but it's an actually really useful field to have for debugging. We should open it up, similar to how we use QueryExecution.

## How was this patch tested?
N/A - this is a simple visibility change.

Author: Reynold Xin <rxin@databricks.com>

Closes #15724 from rxin/SPARK-18216.

sql/core/src/main/scala/org/apache/spark/sql/Column.scala

@@ -118,6 +118,9 @@ class TypedColumn[-T, U](
  *   $"a" === $"b"
  * }}}
  *
+ * Note that the internal Catalyst expression can be accessed via "expr", but this method is for
+ * debugging purposes only and can change in any future Spark releases.
+ *
  * @groupname java_expr_ops Java-specific expression operators
  * @groupname expr_ops Expression operators
  * @groupname df_ops DataFrame functions
@@ -126,7 +129,7 @@ class TypedColumn[-T, U](
  * @since 1.3.0
  */
 @InterfaceStability.Stable
-class Column(protected[sql] val expr: Expression) extends Logging {
+class Column(val expr: Expression) extends Logging {
 
   def this(name: String) = this(name match {
     case "*" => UnresolvedStar(None)