[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