Refactor large object sweeping.

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

Change-Id: I938c1f29f69b0682350347da2bd5de021c0e0224
diff --git a/runtime/gc/space/large_object_space.h b/runtime/gc/space/large_object_space.h
index d374ad3..cd7c383 100644
--- a/runtime/gc/space/large_object_space.h
+++ b/runtime/gc/space/large_object_space.h
@@ -67,6 +67,8 @@
     return this;
   }
 
+  virtual void Sweep(bool swap_bitmaps, size_t* freed_objects, size_t* freed_bytes);
+
  protected:
   explicit LargeObjectSpace(const std::string& name);