Add possibility to query the hidden field of a class that represents the reference count

symdef.pas, tobjectdef:
  + new method "refcount_field" which returns the (potentially inherited) field that holds the reference count or Nil if the class isn't reference counted


git-svn-id: http://svn.freepascal.org/svn/fpc/branches/svenbarth@28808 3ad0048d-3df7-0310-abae-a5850022a9f2