Rollup merge of #83581 - arennow:dir_entry_ext_unix_borrow_name, r=m-ou-se

Add std::os::unix::fs::DirEntryExt2::file_name_ref(&self) -> &OsStr

Greetings!

This is my first PR here, so please forgive me if I've missed an important step or otherwise done something wrong. I'm very open to suggestions/fixes/corrections.

This PR adds a function that allows `std::fs::DirEntry` to vend a borrow of its filename on Unix platforms, which is especially useful for sorting. (Windows has (as I understand it) encoding differences that require an allocation.) This new function sits alongside the cross-platform [`file_name(&self) -> OsString`](https://doc.rust-lang.org/std/fs/struct.DirEntry.html#method.file_name) function.

I pitched this idea in an [internals thread](https://internals.rust-lang.org/t/allow-std-direntry-to-vend-borrows-of-its-filename/14328/4), and no one objected vehemently, so here we are.

I understand features in general, I believe, but I'm not at all confident that my whole-cloth invention of a new feature string (as required by the compiler) was correct (or that the name is appropriate). Further, there doesn't appear to be a test for the sibling `ino` function, so I didn't add one for this similarly trivial function either. If it's desirable that I should do so, I'd be happy to [figure out how to] do that.

The following is a trivial sample of a use-case for this function, in which directory entries are sorted without any additional allocations:

```rust
use std::os::unix::fs::DirEntryExt;
use std::{fs, io};

fn main() -> io::Result<()> {
    let mut entries = fs::read_dir(".")?.collect::<Result<Vec<_>, io::Error>>()?;
    entries.sort_unstable_by(|a, b| a.file_name_ref().cmp(b.file_name_ref()));

    for p in entries {
        println!("{:?}", p);
    }

    Ok(())
}
```

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