Switched AppleObjCRuntimeV2::CreateClassDescriptor

over to simply update its cache and then look up
the descriptor in the cache.  This is fine because
the cache now builds much faster (since descriptors
are minimal).

Metaclasses aren't in the cache, so I switched
the Describe method for class descriptors from using
GetClassDescriptor to manually creating an automatic
ClassDescriptorV2.

llvm-svn: 165579