vim plugins: use v6.0 of the Clang library

clighter8 plugin seems to be using python bindings for clang 6.0
  clang_complete, another plugin using clang, works ok with this version

pkgs/misc/vim-plugins/default.nix

@@ -19,6 +19,7 @@ let
   overrides = callPackage ./overrides.nix {
     inherit (darwin.apple_sdk.frameworks) Cocoa CoreFoundation CoreServices;
     inherit buildVimPluginFrom2Nix;
+    inherit llvmPackages;
   };
 
   overriden = generated // (overrides generated);

pkgs/top-level/all-packages.nix

@@ -22654,8 +22654,8 @@ with pkgs;
 
   vimUtils = callPackage ../misc/vim-plugins/vim-utils.nix { };
 
-  vimPlugins = recurseIntoAttrs (callPackage ../misc/vim-plugins {
-    llvmPackages = llvmPackages_39;
+  vimPlugins = recurseIntoAttrs (callPackage ../misc/vim-plugins { 
+    llvmPackages = llvmPackages_6;
   });
 
   vimprobable2-unwrapped = callPackage ../applications/networking/browsers/vimprobable2 {