Bug#850915: ghc: Please switch to llvm 3.8 or, better, 3.9

[Gianfranco Costamagna]

Hello

>Probably need to backport this upstream commit:

>
>https://git.haskell.org/ghc.git/commitdiff/672314cbeb8ac386a58f17dc4650dbdf4c55d8b5


probably not only that one, but at least 3 commits

Unfortunately when I expressed the idea (and the commits) over irc on #debian-haskell or whatever
I got a nack, because diverging from upstream in the default toolchain version is a no-go,
and moreover it would require probably a lot of binNMUs in the arm* architectures for the whole
haskell stack.

Upstream can fix bugs when llvm 3.7 is used, now when we diverge in such important key packages.

(this is a sum of the discussion I recall right now, I can search irc logs if needed)

G.