Upgrade llvm to current HEAD

* This has one workaround patch (everything's testing just fine...)
* I reworked the fixedstacksegment attribute to be specified with a string
  rather than using a keyword and an integer and modifying the parser
* I added a "no-split-stack" attribute along the same lines as the
  "fixedstacksegment" attribute for #1226
Fix LLVM compilation issues and use the new attrs

This implements #[no_split_stack] and also changes #[fast_ffi] to using the new
"fixedstacksegment" string attribute instead of integer attribute.