Move LLVM code generator to LLVM bitcode format

Currently GHC produces LLVM's textual intermediate representation. While this is easy to produce, it's not the path recommended by LLVM's developers. Instead, they recommend that compilers produce LLVM bitcode, which they maintain much stronger backwards compatibility guarantees on.

It seems sensible to move the backend over to this format.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (LLVM)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

mentioned in issue #10074

changed weight to 5

added LLVM backend Tfeature request Trac import labels

assigned to @trac-angermann

Moritz Angermann is currently looking into this. Yay!

A preview of this is now available as per https://mail.haskell.org/pipermail/ghc-devs/2017-September/014689.html

Trac metadata

Trac field	Value
Version	8.0.1 → 8.3

added Pnormal label