Refine ObjCPropertyImplDecl:

- Remove a bunch of setXXX methods until we actually need them. This cleans up the
interface, and makes the object immutable until we have a reason to be mutable.

- Remove enum value OBJC_PR_IMPL_None since it was never used.

- Remove instance variable 'PropertyImplKind PropertyImplementation;'. This is
  not needed because we can tell if ObjCPropertyImplDecl represents @synthesize
  if 'ObjCPropertyDecl *PropertyDecl' is not null.

llvm-svn: 54055