irony-server: use libclang

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

···
   inherit (irony) version;
 
   nativeBuildInputs = [ cmake ];
+  buildInputs = [ llvmPackages.libclang ];
 
   dontUseCmakeBuildDir = true;
 
   cmakeDir = "server";
 
   cmakeFlags = [
-    ''-DCMAKE_PREFIX_PATH=${llvmPackages.clang.cc}''
+    "-DCMAKE_PREFIX_PATH=${llvmPackages.clang-unwrapped}"
   ];
 
   src = irony.src;