So an Apple engineer spent time adding support for more 'liberal', non-Apple-based Objective-C runtimes? Wow!
That seems unusual of Apple, but very cool.
EDIT - Looking at the clang source, it looks like GNUstep and and ObjFW (with the exception of old GCC) are the only alternative Objective-C runtimes currently supported by clang:
http://clang.llvm.org/doxygen/ObjCRuntime_8h_source.html
Unusual? It'd seem beneficial to Apple in several ways, in that they have multiple ABIs (see the code there!), and in that it keeps other Obj-C users interested in Clang (which means more outside work on the Obj-C frontend).
https://webkeks.org/blog/?5f