I personally prefer to stick to LLVM releases wherever possible, but the update was done to implement unwinding for i686 MSVC which I would personally deem more important than sticking with any one particular LLVM release.
As @huon mentioned we maintain compatibility with currently released versions of LLVM (e.g. 3.5-3.7), and we will merge anything necessary to be compatible with the 3.8 release as well.
Thanks guys for the explanation. I observed some horrible crashes with my locally built i686->x86_64 cross compiler and wondered if there was anything wrong with using the distro LLVM. I’ll post more details as a GitHub issue.