Rollup merge of #81879 - imbrem:make-reverse-repr-transparent, r=m-ou-se

Rollup merge of #81879 - imbrem:make-reverse-repr-transparent, r=m-ou-se

Added #[repr(transparent)] to core::cmp::Reverse

I found casting from an `&T` to an `&Reverse<T>` potentially useful, but found that `Reverse` was not `#[repr(transparent)]`, so after asking about it [on Reddit](https://www.reddit.com/r/rust/comments/le60uv/make_stdcmpreverse_reprtransparent_and_add_a/), I decided to go ahead and make a pull request which simply adds the attribute to the struct.