Rollup merge of #90427 - jkugelman:must-use-alloc-leak, r=joshtriplett

Add #[must_use] to alloc functions that would leak memory

As [requested](https://github.com/rust-lang/rust/pull/89899#issuecomment-955600779) by `@joshtriplett.`

> Please do go ahead and add the ones whose only legitimate use for ignoring the return value is leaking memory. (In a separate PR please.) I think it's sufficiently error-prone to call something like alloc and ignore the result that it's legitimate to require `let _ =` for that.

I added `realloc` myself. Clippy ignored it because of its `mut` argument.

```rust
alloc/src/alloc.rs:123:1   alloc   unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8;
```

Parent issue: #89692

r? `@joshtriplett`

library/alloc/src/alloc.rs

@@ -81,6 +81,7 @@ pub use std::alloc::Global;
 /// }
 /// ```
 #[stable(feature = "global_alloc", since = "1.28.0")]
+#[must_use = "losing the pointer will leak memory"]
 #[inline]
 pub unsafe fn alloc(layout: Layout) -> *mut u8 {
     unsafe { __rust_alloc(layout.size(), layout.align()) }
@@ -117,6 +118,7 @@ pub unsafe fn dealloc(ptr: *mut u8, layout: Layout) {
 ///
 /// See [`GlobalAlloc::realloc`].
 #[stable(feature = "global_alloc", since = "1.28.0")]
+#[must_use = "losing the pointer will leak memory"]
 #[inline]
 pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
     unsafe { __rust_realloc(ptr, layout.size(), layout.align(), new_size) }
@@ -150,6 +152,7 @@ pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8
 /// }
 /// ```
 #[stable(feature = "global_alloc", since = "1.28.0")]
+#[must_use = "losing the pointer will leak memory"]
 #[inline]
 pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
     unsafe { __rust_alloc_zeroed(layout.size(), layout.align()) }

library/alloc/src/sync.rs

@@ -804,6 +804,7 @@ impl<T: ?Sized> Arc<T> {
     /// let x_ptr = Arc::into_raw(x);
     /// assert_eq!(unsafe { &*x_ptr }, "hello");
     /// ```
+    #[must_use = "losing the pointer will leak memory"]
     #[stable(feature = "rc_raw", since = "1.17.0")]
     pub fn into_raw(this: Self) -> *const T {
         let ptr = Self::as_ptr(&this);