[SQL] Add type checking debugging functions

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.

Author: Michael Armbrust <michael@databricks.com>

Closes #2657 from marmbrus/debugging and squashes the following commits:

654b926 [Michael Armbrust] Clean-up, add tests
763af15 [Michael Armbrust] Add typeChecking debugging functions
8c69303 [Michael Armbrust] Add inputSet, references to QueryPlan. Improve tree string with a prefix to denote invalid or unresolved nodes.
fbeab54 [Michael Armbrust] Better toString, factories for AttributeSet.