[SPARK-6459][SQL] Warn when constructing trivially true equals predicate

For example, one might expect the following code to work, but it does not.  Now you will at least get a warning with a suggestion to use aliases.

```scala
val df = sqlContext.load(path, "parquet")
val txns = df.groupBy("cust_id").agg($"cust_id", countDistinct($"day_num").as("txns"))
val spend = df.groupBy("cust_id").agg($"cust_id", sum($"extended_price").as("spend"))
val rmJoin = txns.join(spend, txns("cust_id") === spend("cust_id"), "inner")
```

Author: Michael Armbrust <michael@databricks.com>

Closes #5163 from marmbrus/selfJoinError and squashes the following commits:

16c1f0b [Michael Armbrust] fix visibility
1b57e8d [Michael Armbrust] Warn when constructing trivially true equals predicate

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

@@ -20,6 +20,7 @@ package org.apache.spark.sql
 import scala.language.implicitConversions
 
 import org.apache.spark.annotation.Experimental
+import org.apache.spark.Logging
 import org.apache.spark.sql.functions.lit
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.analysis.{UnresolvedAttribute, UnresolvedStar, UnresolvedGetField}
@@ -46,7 +47,7 @@ private[sql] object Column {
  * @groupname Ungrouped Support functions for DataFrames.
  */
 @Experimental
-class Column(protected[sql] val expr: Expression) {
+class Column(protected[sql] val expr: Expression) extends Logging {
 
   def this(name: String) = this(name match {
     case "*" => UnresolvedStar(None)
@@ -109,7 +110,15 @@ class Column(protected[sql] val expr: Expression) {
    *
    * @group expr_ops
    */
-  def === (other: Any): Column = EqualTo(expr, lit(other).expr)
+  def === (other: Any): Column = {
+    val right = lit(other).expr
+    if (this.expr == right) {
+      logWarning(
+        s"Constructing trivially true equals predicate, '${this.expr} = $right'. " +
+          "Perhaps you need to use aliases.")
+    }
+    EqualTo(expr, right)
+  }
 
   /**
    * Equality test.