Summary:
Rework llvmGen to use LLVM 3.6 exclusively. The plans for the 7.12 release are to ship LLVM alongside GHC in the interests of user (and developer) sanity.

Along the way, refactor TNTC support to take advantage of the new `prefix` data support in LLVM 3.6. This allows us to drop the section-reordering component of the LLVM mangler.

Test Plan: Validate, look at emitted code

Reviewers: dterei, austin, scpmw

Reviewed By: austin

Subscribers: erikd, awson, spacekitteh, thomie, carter

Differential Revision: https://phabricator.haskell.org/D530

GHC Trac Issues: #10074