[SPARK-10475][SQL] improve column prunning for Project on Sort

[cloud-fan]

Sometimes we can't push down the whole Project though Sort, but we still have a chance to push down part of it.

[SparkQA]

Test build #42101 has finished for PR 8644 at commit 79415f4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

cc @marmbrus

[marmbrus]

Thanks, merging to master.