Refactor ParsedColInfo to be more statement-neutral

I want to be able to have an ORDER BY clause on a DELETE statement.  In our planner we represent the list of things following ORDER BY as instances of ParsedColInfo.

On master this class in nested with ParsedSelectStmt, and in general is pretty SELECT-centric.  This branch un-nests the class, and attempts to provide a bit more encapsulation.  This class could use some more refactoring (proper constructor, make members private), but it's got lots of uses all over the planner and DDL compiler, so I didn't do all of this.  This is a little better at least.