Sema: Split up gatherConstraints() into gatherAllConstraints() and gatherNearbyConstraints()

The two GatherKinds no longer share any implementation, so there's
no point keeping the logic together. Doing this also allows removing
the acceptConstraintFn from gatherAllConstraints(), which further
simplifies depthFirstSearch().

lib/Sema/CSSimplify.cpp

@@ -15953,8 +15953,8 @@ ConstraintSystem::addKeyPathApplicationRootConstraint(Type root, ConstraintLocat
   if (!typeVar)
     return;
 
-  auto constraints = CG.gatherConstraints(
-      typeVar, ConstraintGraph::GatheringKind::EquivalenceClass,
+  auto constraints = CG.gatherNearbyConstraints(
+      typeVar,
       [&keyPathExpr](Constraint *constraint) -> bool {
         if (constraint->getKind() != ConstraintKind::KeyPath)
           return false;