Rollup merge of rust-lang#91796 - not-my-profile:fix-const_manually_drop-since, r=kennytm

Fix since attribute for const_manually_drop feature

const_manually_drop was stabilized in 1.32 as mentioned in
https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1320-2019-01-17

Author: matthiaskrgr

library/core/src/mem/manually_drop.rs

@@ -64,7 +64,7 @@ impl<T> ManuallyDrop<T> {
     /// ```
     #[must_use = "if you don't need the wrapper, you can use `mem::forget` instead"]
     #[stable(feature = "manually_drop", since = "1.20.0")]
-    #[rustc_const_stable(feature = "const_manually_drop", since = "1.36.0")]
+    #[rustc_const_stable(feature = "const_manually_drop", since = "1.32.0")]
     #[inline(always)]
     pub const fn new(value: T) -> ManuallyDrop<T> {
         ManuallyDrop { value }
@@ -82,7 +82,7 @@ impl<T> ManuallyDrop<T> {
     /// let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.
     /// ```
     #[stable(feature = "manually_drop", since = "1.20.0")]
-    #[rustc_const_stable(feature = "const_manually_drop", since = "1.36.0")]
+    #[rustc_const_stable(feature = "const_manually_drop", since = "1.32.0")]
     #[inline(always)]
     pub const fn into_inner(slot: ManuallyDrop<T>) -> T {
         slot.value