Rollup merge of rust-lang#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.

Author: m-ou-se

library/core/src/cmp.rs

@@ -579,6 +579,7 @@ impl Ordering {
 /// ```
 #[derive(PartialEq, Eq, Debug, Copy, Clone, Default, Hash)]
 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
+#[repr(transparent)]
 pub struct Reverse<T>(#[stable(feature = "reverse_cmp_key", since = "1.19.0")] pub T);
 
 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]