Stabilize alloc::Layout const functions

Stabilizes #67521. In particular the following stable methods are stabilized as const fn:

* size
* align
* from_size_align

library/core/src/alloc/layout.rs

@@ -50,7 +50,7 @@ impl Layout {
     ///    must not overflow (i.e., the rounded value must be less than
     ///    or equal to `usize::MAX`).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
         if !align.is_power_of_two() {
@@ -96,15 +96,15 @@ impl Layout {
 
     /// The minimum size in bytes for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn size(&self) -> usize {
         self.size_
     }
 
     /// The minimum byte alignment for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn align(&self) -> usize {
         self.align_.get()