[SPARK-7303] [SQL] push down project if possible when the child is sort

Optimize the case of `project(_, sort)` , a example is:

`select key from (select * from testData order by key) t`

before this PR:
```
== Parsed Logical Plan ==
'Project ['key]
 'Subquery t
  'Sort ['key ASC], true
   'Project [*]
    'UnresolvedRelation [testData], None

== Analyzed Logical Plan ==
Project [key#0]
 Subquery t
  Sort [key#0 ASC], true
   Project [key#0,value#1]
    Subquery testData
     LogicalRDD [key#0,value#1], MapPartitionsRDD[1]

== Optimized Logical Plan ==
Project [key#0]
 Sort [key#0 ASC], true
  LogicalRDD [key#0,value#1], MapPartitionsRDD[1]

== Physical Plan ==
Project [key#0]
 Sort [key#0 ASC], true
  Exchange (RangePartitioning [key#0 ASC], 5), []
   PhysicalRDD [key#0,value#1], MapPartitionsRDD[1]
```

after this PR
```
== Parsed Logical Plan ==
'Project ['key]
 'Subquery t
  'Sort ['key ASC], true
   'Project [*]
    'UnresolvedRelation [testData], None

== Analyzed Logical Plan ==
Project [key#0]
 Subquery t
  Sort [key#0 ASC], true
   Project [key#0,value#1]
    Subquery testData
     LogicalRDD [key#0,value#1], MapPartitionsRDD[1]

== Optimized Logical Plan ==
Sort [key#0 ASC], true
 Project [key#0]
  LogicalRDD [key#0,value#1], MapPartitionsRDD[1]

== Physical Plan ==
Sort [key#0 ASC], true
 Exchange (RangePartitioning [key#0 ASC], 5), []
  Project [key#0]
   PhysicalRDD [key#0,value#1], MapPartitionsRDD[1]
```

with this rule we will first do column pruning on the table and then do sorting.

Author: scwf <wangfei1@huawei.com>

This patch had conflicts when merged, resolved by
Committer: Michael Armbrust <michael@databricks.com>

Closes #5838 from scwf/pruning and squashes the following commits:

b00d833 [scwf] address michael's comment
e230155 [scwf] fix tests failure
b09b895 [scwf] improve column pruning