irony-server: fix build, fix llvm references

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

···
   pname = "irony-server";
   inherit (irony) src version;
 
-  nativeBuildInputs = [ cmake ];
+  nativeBuildInputs = [ cmake llvmPackages.llvm.dev ];
   buildInputs = [ llvmPackages.libclang llvmPackages.llvm ];
 
   dontUseCmakeBuildDir = true;