Rollup merge of #137105 - zachs18:cow-derefpure-restrict, r=Nadrieril

Restrict DerefPure for Cow<T> impl to T = impl Clone, [impl Clone], str.

Fixes #136046

`feature(deref_patterns)` tracking issue: rust-lang/rust#87121

`Cow<'_, T>` should only implement `DerefPure` if its `Deref` impl is pure, which requires `<T::Owned as Borrow<T>>::borrow`  to be pure. This PR restricts `impl DerefPure for Cow<'_, T>` to `T: Sized + Clone`, `T = [U: Clone]`, and `T = str` (for all of whom `<T::Owned as Borrow<T>>::borrow` is implemented in the stdlib and is pure).

cc ``@Nadrieril``

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An alternate approach would be to introduce a new `unsafe trait BorrowPure<T>` analogous to `DerefPure`  that could be implemented for `T: Sized`, `&T`, `&mut T`, `String`, `Vec`, `Box`, `PathBuf`, `OsString`, etc. rust-lang/rust@master...zachs18:borrow-pure-trait

Author: matthiaskrgr