[SPARK-6376][SQL] Avoid eliminating subqueries until optimization

Previously it was okay to throw away subqueries after analysis, as we would never try to use that tree for resolution again.  However, with eager analysis in `DataFrame`s this can cause errors for queries such as:

```scala
val df = Seq(1,2,3).map(i => (i, i.toString)).toDF("int", "str")
df.as('x).join(df.as('y), $"x.str" === $"y.str").groupBy("x.str").count()
```

As a result, in this PR we defer the elimination of subqueries until the optimization phase.

Author: Michael Armbrust <michael@databricks.com>

Closes #5160 from marmbrus/subqueriesInDfs and squashes the following commits:

a9bb262 [Michael Armbrust] Update Optimizer.scala
27d25bf [Michael Armbrust] fix hive tests
9137e03 [Michael Armbrust] add type
81cd597 [Michael Armbrust] Avoid eliminating subqueries until optimization