Hey Guix,

here's a patch to update Mesa on staging to the just released 18.1.1.
Since there's a little more involved than usual I'm sending it in for
review first.

The 18.1 series needs to be built with LLVM >= 4 (the current
18.2-devel series needs to be built with LLVM >=5). As such I've
changed Mesa to use LLVM 6.0.

Feedback/testing welcome!