[SceneKit] Add apinotes to swift_private some decls

These apinotes will swift_private many of the bounding box methods,
and adjust the overlays appropriately. Those APIs have better
alternatives provided by the overlays, and thus shouldln't be exposed.

stdlib/public/SDK/SceneKit/SceneKit.swift

@@ -199,19 +199,19 @@ extension SCNBoundingVolume {
     get {
       var min = SCNVector3Zero
       var max = SCNVector3Zero
-      getBoundingBoxMin(&min, max: &max)
+      __getBoundingBoxMin(&min, max: &max)
       return (min: min, max: max)
     }
     set {
       var min = newValue.min
       var max = newValue.max
-      setBoundingBoxMin(&min, max: &max)
+      __setBoundingBoxMin(&min, max: &max)
     }
   }
   public var boundingSphere: (center: SCNVector3, radius: Float) {
     var center = SCNVector3Zero
     var radius = CGFloat(0.0)
-    getBoundingSphereCenter(&center, radius: &radius)
+    __getBoundingSphereCenter(&center, radius: &radius)
     return (center: center, radius: Float(radius))
   }
 }