Remove more owned raw pointers from QueryPlanner code

The QueryPlanner still has places where it uses owned raw pointers which should be modernized to use std::unique_ptr:

• QuerySolutionNode::clone() returns a raw pointer owned by the caller.
• QueryPlannerAnalysis::analyzeSort() returns a raw pointer to a QuerySolutionNode owned by the caller, and holds the current root node as a raw pointer.

QuerySolutionNode also holds its children by raw pointer rather than std::unique_ptr, but fixing this is tracked by related ticket SERVER-35512.