Split Quantified and PartialQuantified code paths

Summary:
For #105.

PartialQuantified will continue to have eager pinning behavior, whereas we want to delay solving Quantified in some cases. So let's split these code paths to make later changes easier.

This diff does not change any user-facing behavior.

Reviewed By: migeed-z

Differential Revision: D96334059

fbshipit-source-id: 5543a6db71879831d902a0c2b80cb06bacaccf38

Author: rchen152

pyrefly/lib/solver/solver.rs

@@ -2013,25 +2013,24 @@ impl<'a, Ans: LookupAnswer> Subset<'a, Ans> {
                         } else {
                             t1_p.clone()
                         };
-                        self.solver
-                            .variables
-                            .lock()
-                            .update(*v2, Variable::Answer(answer));
                         // Widen None to None | Any for PartialQuantified, matching
                         // the PartialContained behavior (see comment there).
                         if is_partial {
                             let variables = self.solver.variables.lock();
-                            let v2_current = variables.get(*v2);
-                            if let Variable::Answer(t) = &*v2_current
-                                && t.is_none()
-                            {
+                            if answer.is_none() {
                                 let widened = self
                                     .solver
                                     .heap
-                                    .mk_union(vec![t.clone(), Type::any_implicit()]);
-                                drop(v2_current);
+                                    .mk_union(vec![answer.clone(), Type::any_implicit()]);
                                 variables.update(*v2, Variable::Answer(widened));
+                            } else {
+                                variables.update(*v2, Variable::Answer(answer));
                             }
+                        } else {
+                            self.solver
+                                .variables
+                                .lock()
+                                .update(*v2, Variable::Answer(answer));
                         }
                         Ok(())
                     }