clang8: apply patch to disable xpc

pkgs/development/compilers/llvm/8/clang/default.nix

@@ -24,6 +24,7 @@ let
 
     cmakeFlags = [
       "-DCMAKE_CXX_FLAGS=-std=c++11"
+      "-DCLANGD_BUILD_XPC=OFF"
     ] ++ stdenv.lib.optionals enableManpages [
       "-DCLANG_INCLUDE_DOCS=ON"
       "-DLLVM_ENABLE_SPHINX=ON"
@@ -35,7 +36,10 @@ let
       "-DLINK_POLLY_INTO_TOOLS=ON"
     ];
 
-    patches = [ ./purity.patch ];
+    patches = [
+      ./purity.patch
+      ./clang-xpc.patch
+    ];
 
     postPatch = ''
       sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \