Refactor large object sweeping.

Moved basic sweeping logic into large_object_space.cc.
Renamed SpaceSetMap -> ObjectSet.

Change-Id: I938c1f29f69b0682350347da2bd5de021c0e0224
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index 8d607e2..071b3de 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -1440,7 +1440,7 @@
 
 void Heap::MarkAllocStack(accounting::SpaceBitmap* bitmap1,
                           accounting::SpaceBitmap* bitmap2,
-                          accounting::SpaceSetMap* large_objects,
+                          accounting::ObjectSet* large_objects,
                           accounting::ObjectStack* stack) {
   DCHECK(bitmap1 != nullptr);
   DCHECK(bitmap2 != nullptr);