Rebases llvm to near-head, fixes .ll parse errors with fixedstacksegment attribute

Good morning,

The `fixedstacksegment` patch left out updating the `.ll` parser to handle the new attribute. I've added it, and I hope I did it correctly. This also rebases our patches to near the top of the llvm tree. It works fine on my mac, but I haven't yet tested it on the other OSs yet. I wanted to make sure this got in before https://github.com/mozilla/rust/issues/4259#issuecomment-14613876 pushes the changes upstream.