[clangd] Use TimePoint<> instead of system_clock::time_point, it does matter after all.

  // Time when the cache was known valid (reflected disk state).

  mutable std::chrono::steady_clock::time_point ValidTime;

  // Filesystem metadata corresponding to the currently cached data.

  mutable std::chrono::system_clock::time_point ModifiedTime;

  mutable llvm::sys::TimePoint<> ModifiedTime;

  mutable uint64_t Size;

};