Auto merge of #96232 - sunfishcode:sunfishcode/io-safety-const-fns, r=joshtriplett

Make `BorrowedFd::borrow_raw` a const fn.

Making `BorrowedFd::borrow_raw` a const fn allows it to be used to
create a constant `BorrowedFd<'static>` holding constants such as
`AT_FDCWD`. This will allow [`rustix::fs::cwd`] to become a const fn.

For consistency, make similar changes to `BorrowedHandle::borrow_raw`
and `BorrowedSocket::borrow_raw`.

[`rustix::fs::cwd`]: https://docs.rs/rustix/latest/rustix/fs/fn.cwd.html

r? `@joshtriplett`

Author: bors