llvm 3.8, 3.9, 4: WIP split out compiler-rt for other versions

[Ericson2314]

Motivation for this change

This is good for build parallelism and cross compilation.

6 was already done, 5 is is Darwin's stdenv so holding off to avoid mass
rebuild, <3.9 is too old for separate compiler-rt.

Things done

There are some build failures to track down, however. I'm opening this so others can possibly assist.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

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[matthewbauer]

Let's hold off on merging any of this until 18.09 is branched off.

[Ericson2314]

@matthewbauer well on the other hand we already did this with the far more important LLVMs, without downstream issues IIRC, so if I can debug the two build failures here (I got locally) it should be a master-only PR that gets everything into a consistent state for the release.

[LnL7]

Wouldn't llvm-5 be inconsistent with all the other version then?

change

[matthewbauer]

Ok fair enough.

[Ericson2314]

There's two failures on Linux, and I cannot currently test Darwin. Help with this would be appreciated.

[vcunat]

This is dead?

[Ericson2314]

I just never finished debugging it! Maybe I can soon now that I have access to beefier machines :).

[Gaelan]

The LLVM versions in question are all gone now, and it looks like this is done for all of the versions currently in Nixpkgs. This can probably be closed.