https://bugs.freedesktop.org/show_bug.cgi?id=99013
--- Comment #10 from Marek Olšák maraeo@gmail.com --- Mesa only checks for LLVM 4.0. There is no distinction between LLVM 4.0 and 4.0svn.
If you want LLVM git, you always have to update Mesa from git along with LLVM and hope for the best. Or you can stick with LLVM 3.9 and only use Mesa git.