Auto merge of rust-lang#117537 - GKFX:offset-of-enum-feature, r=cjgillot

Feature gate enums in offset_of

As requested at rust-lang#106655 (comment), put enums in offset_of behind their own feature gate.

`@rustbot` label F-offset_of

Author: bors

core/src/mem/mod.rs

@@ -1358,6 +1358,7 @@ impl<T> SizedTypeProperties for T {}
 ///
 /// ```
 /// #![feature(offset_of)]
+/// # #![cfg_attr(not(bootstrap), feature(offset_of_enum))]
 ///
 /// use std::mem;
 /// #[repr(C)]