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.
auto merge of #8328 : alexcrichton/rust/llvm-head, r=brson

The first commit message is pretty good, but whomever reviews this should probably also at least glance at the changes I made in LLVM. I basically reorganized our pending patch queue to be a bit more organized and clearer in what needs to go where. After this, our queue would be:

* Add the `no-split-stack` attribute
* Add the `fixedstacksegment` attribute
* Add split-stacks for arm android
* Add split-stacks for arm linux
* Add split stacks for mips

Then there's a patch which I added to get rust to build at all on LLVM-head, and I'm not quite sure why it's there, but nothing seems to be crashing for now! (famous last words).

Otherwise, I just updated code to reflect the changes I made in LLVM with the only major change being the advent of the new `no_split_stack` attribute. This is work towards #1226, but someone more familiar with the code should probably actually assign the attribute to the appropriate functions.

Also as a bonus, I've verified that this closes #5774