[libc++] Match declaration for non-member function...

[libc++] Match declaration for non-member function std::swap(std::packaged_task) with what standard specify

Standard specifies:
```
template<class R, class... ArgTypes>
  void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
```

Differential Revision: https://reviews.llvm.org/D99102