DO NOT pre-defined __OBJC2__. The __OBJC2__ macro should only be defined when...

DO NOT pre-defined __OBJC2__. The __OBJC2__ macro should only be defined when targeting the new, Apple 2.0 *runtime ABI*. It is not intended to be used to #ifdef ObjC 2.0 langauge features. This is unfortunate (given it's name). In a perfect world, this defined would be named __OBJC2_RUNTIME_ABI__. Oh well.

llvm-svn: 50913