[libc++] P2770R0: Stashing stashing iterators for proper flattening (#66033)

- Partially implements P2770R0 (http://wg21.link/p2770)
- Fixes https://wg21.link/LWG3698, https://wg21.link/LWG3700, and https://wg21.link/LWG3791
- join_with_view hasn't been done yet since this type isn't implemented yet
- Rename tuple test directory to match the standard (which changed in P2770R0)
- Rename join_view test directory to match the standard