Mark Layout::new as const stable

Author: CAD97

src/libcore/alloc.rs

@@ -120,6 +120,7 @@ impl Layout {
 
     /// Constructs a `Layout` suitable for holding a value of type `T`.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
+    #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
     #[inline]
     pub const fn new<T>() -> Self {
         let (size, align) = size_align::<T>();