[SQL] Add type checking debugging functions

[marmbrus]

Adds some functions that were very useful when trying to track down the bug from #2656. This change also changes the tree output for query plans to include the ' prefix to unresolved nodes and ! prefix to nodes that refer to non-existent attributes.

[SparkQA]

QA tests have started for PR 2657 at commit 1d0c2da.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2657 at commit 1d0c2da.

• This patch fails unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class TypeCheck(child: SparkPlan) extends SparkPlan

[SparkQA]

QA tests have started for PR 2657 at commit 763af15.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2657 at commit 763af15.

• This patch fails unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class TypeCheck(child: SparkPlan) extends SparkPlan

[SparkQA]

QA tests have started for PR 2657 at commit 654b926.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2657 at commit 654b926.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

QA tests have started for PR 2657 at commit 654b926.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2657 at commit 654b926.

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

[jiangxb1987]

Perhaps we should use toAttribute instead of references for NamedExpression? Otherwise the AttributeSet.contains method may return false result. IMO we should seprate the purpose of cacheing expressions as attributes from cacheing expression.refercences.