The requirements for this function are only that the deleter is
swappable, but we incorrectly require that the element type is complete
and that the deleter can be swapped using std::swap (which requires it
to be move cosntructible and move assignable).
The fix is to add __uniq_ptr_impl::swap which swaps the pointer and
deleter individually, instead of using the generic std::swap on the
tuple containing them.
PR libstdc++/93562
* include/bits/unique_ptr.h (__uniq_ptr_impl::swap): Define.
(unique_ptr::swap, unique_ptr<T[], D>::swap): Call it.
* testsuite/20_util/unique_ptr/modifiers/93562.cc: New test.
