[SPARK-20073][SQL] Prints an explicit warning message in case of NULL-safe equals

## What changes were proposed in this pull request?
This pr added code to print the same warning messages with `===` cases when using NULL-safe equals (`<=>`).

## How was this patch tested?
Existing tests.

Author: Takeshi Yamamuro <yamamuro@apache.org>

Closes #18436 from maropu/SPARK-20073.

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

@@ -464,7 +464,15 @@ class Column(val expr: Expression) extends Logging {
    * @group expr_ops
    * @since 1.3.0
    */
-  def <=> (other: Any): Column = withExpr { EqualNullSafe(expr, lit(other).expr) }
+  def <=> (other: Any): Column = withExpr {
+    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.")
+    }
+    EqualNullSafe(expr, right)
+  }
 
   /**
    * Equality test that is safe for null values.