Swap usize -> ptr transmute for strict_pov API

This is the only such conversion left in `alloc`. Didn't trip the
`fuzzy_provenance_casts` lint because it's a transmute.

Author: jwnrt

library/alloc/src/raw_vec/mod.rs

@@ -411,7 +411,7 @@ impl<A: Allocator> RawVecInner<A> {
     #[inline]
     const fn new_in(alloc: A, align: Alignment) -> Self {
         // SAFETY: `Alignment` is non-zero.
-        let ptr = unsafe { core::mem::transmute(align) };
+        let ptr = unsafe { Unique::new_unchecked(ptr::without_provenance_mut(align.as_usize())) };
         // `cap: 0` means "unallocated". zero-sized types are ignored.
         Self { ptr, cap: ZERO_CAP, alloc }
     }