Auto merge of #124640 - Billy-Sheppard:master, r=dtolnay

Fix #124275: Implemented Default for `Arc<str>`

With added implementations.

```
GOOD    Arc<CStr>
BROKEN  Arc<OsStr> // removed
GOOD    Rc<str>
GOOD    Rc<CStr>
BROKEN  Rc<OsStr> // removed

GOOD    Rc<[T]>
GOOD    Arc<[T]>
```

For discussion of rust-lang/rust#124367 (comment).

Key pain points currently:
> I've had a guess at the best locations/feature attrs for them but they might not be correct.

> However I'm unclear how to get the OsStr impl to compile, which file should they go in to avoid the error below? Is it possible, perhaps with some special std rust lib magic?

Author: bors