This may not be how we want to implement class equality checks in the
long run, but in the short term this is _very_ handy. See also:
<rdar://problem/12939898> ER: formalize root classes
This also removes support for greater-than/less-than on Vectors, which
makes no sense given that this has everything to do with the randomness
of the memory allocator and nothing intrinsic about Vectors or class
references in general.
Swift SVN r3628
