Thinned the AppleObjCRuntimeV2's class descriptors.

The following are now derived lazily:

- The name of the class (cached);
- the instance size of the class (not cached);

The following have been removed entirely:

- Whether the class is realized.  This is an
  implementation detail.
- The contents of the objc_class object.  That
  object can be read as needed.
- Whether the class is valid.  The fact that
  we vended a class to begin with means it's
  valid.  We will only give up looking parts
  of it up if they are not in the format we
  expect.

llvm-svn: 165567