haskellPackages.llvm-hs: fix build

It requires llvm_8

pkgs/development/haskell-modules/configuration-nix.nix

@@ -280,7 +280,8 @@ self: super: builtins.intersectAttrs super {
                             then dontCheck
                             else pkgs.lib.id;
       in dontCheckDarwin (super.llvm-hs.override {
-        llvm-config = pkgs.llvm_6;
+        llvm-config = pkgs.llvm_8;
+        llvm-hs-pure = super.llvm-hs-pure_8_0_0;
       });
 
   # Needs help finding LLVM.