Comment # 10 on bug 99013 from Marek Olšák

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.