message: Adapt D73392 - split of llvm/Support/Allocator.h
diff: -// Copyright 2017-2018 ccls Authors
  +// Copyright 2017-2020 ccls Authors
  // SPDX-License-Identifier: Apache-2.0
  #include "serializer.hh"
  #include <llvm/ADT/CachedHashString.h>
  #include <llvm/ADT/DenseSet.h>
  +#include <llvm/Support/Allocator.h>
  #include <mutex>
  #include <stdexcept>
  