[libc++] Change forward_list::swap to use propagate_on_container_swap for noexcept specification

The current implementation of `std::forward_list::swap` uses
`propagate_on_container_move_assignment` for `noexcept` specification.
This patch changes it to use `propagate_on_container_swap`, as it should.

Fixes https://llvm.org/PR50224.

Differential Revision: https://reviews.llvm.org/D101899