[libc++] [P1164] [C++20] Make fs::create_directory() error if there is already a non-directory.

Also mark LWG2935 and LWG3079 as complete.

Applied retroactively to previous standards too, as it's a DR.

* https://wg21.link/P1164
* https://wg21.link/lwg2935
* https://wg21.link/lwg3079

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D92769