Use the diff planner for deriving callee functions.
Recursive derivation is not allowed.
'FunctionDecl* Derivative = Derivative' is ambiguous.
Handle the cases when we derive class methods and struct functions.
Teach the Stmt cloner to clone UnresolvedLookupExpr.
Clone also the default arguments if present.
Exit early if nothing found by the lookup.
Every stmt visit is done using VisitStmt.
VisitStmt serves as a dispatcher sending the stmt to its corresponding cloner.
Improve code readability by using getAs instead of getStmt.
Update tests results.