sim: Make the drain state a global typed enum

The drain state enum is currently a part of the Drainable
interface. The same state machine will be used by the DrainManager to
identify the global state of the simulator. Make the drain state a
global typed enum to better cater for this usage scenario.

src/sim/sim_object.cc

@@ -183,7 +183,7 @@ debugObjectBreak(const char *objs)
 unsigned int
 SimObject::drain(DrainManager *drain_manager)
 {
-    setDrainState(Drained);
+    setDrainState(DrainState::Drained);
     return 0;
 }