Adding a recipe for llvm-mode.

llvm-mode provides syntax highlighting to .ll files, with are LLVM IR files. I'm not a maintainer, this is just me adding the official upstream git repo.

I'm pretty sure that this recipe is correct, but MELPA does a full clone rather than a shallow clone and it's very slow for LLVM's large git repo. I don't see any reason why MELPA couldn't do a shallow clone, the patch is straightforward as far as I can see:

    (pb/run-process nil "git" "clone" repo dir "--depth" "1")

Let me know if I've missed anything.