RFC: Move to LLVM 3.6

Summary:
Here we rework the LLVM backend to support LLVM 3.6 exclusively and refactor
TNTC to take advantage of LLVM's new and improved prefix data support..

Test Plan: Validate, look at emitted code

Reviewers: dterei, austin, scpmw

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D530
Kill unused binding
Fix lints