ObjectMonitor::object() cleanup changes extracted from JDK-8247281

This sub-task is tracking ObjectMonitor::object() cleanup changes
extracted from Erik's work on JDK-8247281. This extraction is done
to ease the code review for the JDK-8247281 changes.

Here's the core cleanup:

diff -r fd7f6a424cd1 src/hotspot/share/runtime/objectMonitor.hpp
--- a/src/hotspot/share/runtime/objectMonitor.hpp Fri Aug 28 16:43:09 2020 -0400
+++ b/src/hotspot/share/runtime/objectMonitor.hpp Wed Sep 02 17:22:56 2020 -0400
@@ -328,9 +328,9 @@

  public:

- void* object() const;
- void* object_addr();
- void set_object(void* obj);
+ oop object() const;
+ oop* object_addr();
+ void set_object(oop obj);
   void release_set_allocation_state(AllocationState s);
   void set_allocation_state(AllocationState s);
   AllocationState allocation_state() const;

and those type changes ripple into the other files.