Auto merge of #138951 - jwnrt:alloc-raw-vec-strict-prov, r=Noratrieb

Replace last `usize` -> `ptr` transmute in `alloc` with strict provenance API

This replaces the `usize -> ptr` transmute in `RawVecInner::new_in` with a strict provenance API (`NonNull::without_provenance`).

The API is changed to take an `Alignment` which encodes the non-null constraint needed for `Unique` and allows us to do the construction safely.

Two internal-only APIs were added to let us avoid UB-checking in this hot code: `Layout::alignment` to get the `Alignment` type directly rather than as a `usize`, and `Unique::from_non_null` to create `Unique` in const context without a transmute.

Author: bors