[libcxx][NFC] removes `swap`'s dependency on `swap_ranges`

Under the as-if rule, we can directly implement the array overload for
`std::swap`. By removing this circular dependency where `swap` is
implemented in terms of `swap_ranges` and `swap_ranges` is defined in
terms of `swap`, we can split them into their own headers. This will:

* limit the surface area in which Hyrum's law can bite us;
* force users to include the correct headers;
* make finding the definitions trivial (`swap` is a utility;
  `swap_ranges` is an algorithm).

Differential Revision: https://reviews.llvm.org/D104760