Auto merge of rust-lang#80707 - oli-obk:stability_hole_const_intrinsics, r=RalfJung

Stability oddity with const intrinsics

cc `@RalfJung`

In rust-lang#80699 (comment) `@usbalbin` realized we accepted some intrinsics as `const` without a `#[rustc_const_(un)stable]` attribute. I did some digging, and that example works because intrinsics inherit their stability from their parents... including `#[rustc_const_(un)stable]` attributes. While we may want to fix that (not sure, wasn't there just a MCPed PR that caused this on purpose?), we definitely want tests for it, thus this PR adding tests and some fun tracing statements.

Author: bors

core/src/intrinsics.rs

@@ -1842,6 +1842,7 @@ pub(crate) fn is_nonoverlapping<T>(src: *const T, dst: *const T, count: usize) -
 #[inline]
 pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize) {
     extern "rust-intrinsic" {
+        #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
         fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
     }
 
@@ -1926,6 +1927,7 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
 #[inline]
 pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
     extern "rust-intrinsic" {
+        #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
         fn copy<T>(src: *const T, dst: *mut T, count: usize);
     }