Trying to build gives a bunch of errors like this:
../../buildtools/third_party/libc++abi/trunk/src/cxa_personality.cpp:1051:23: error: no member named 'barrier_cache' in '_Unwind_Exception'
    unwind_exception->barrier_cache.bitpattern[0] = (uint32_t)results.adjustedPtr;

This is because the unwind.h the compiler is using is from the sysroot.  It needs to be form libunwind:
https://chromium.googlesource.com/external/llvm.org/libunwind/