While most platforms provide a system unwinder for stacks, Apple is contributing to LLVM their unwinder implementation developed for OS X and iOS. This work will go into the libcxxabi project to make it easier to port Clang C++ compiler support to new platforms. For those unfamiliar with stack unwinding, see the information available on lubunwind's man page.
Apple's unwind code uses the same low-level APIs for the unwind functions but do not share code with the old libunwind. Apple pushed their unwinder code into the LLVM libcxxabi sub-project and is under the LLVM and MIT licenses.
More information on LLVM's new unwinder can be found by this LLVM.org blog post, the libcxxabi project page, and the Git commit adding the unwinder.