irony-server: don't use wrapped clang

pkgs/development/tools/irony-server/default.nix

···
   name = "irony-server-${version}";
   inherit (irony) version;
 
-  buildInputs = [ cmake llvmPackages.clang ];
+  nativeBuildInputs = [ cmake ];
 
   dontUseCmakeBuildDir = true;