Rollup merge of #60370 - Richard-W:const-layout-construction, r=sfackler

Mark core::alloc::Layout::from_size_align_unchecked const

Makes it possible (pending stabilization of #57563 (`const_fn`)) to rewrite code like

```rust
const BUFFER_SIZE: usize = 0x2000;
const BUFFER_ALIGN: usize = 0x1000;

fn foo() {
  let layout = std::alloc::Layout::from_size_align(BUFFER_SIZE, BUFFER_ALIGN)
    .unwrap();
  let buffer = std::alloc::alloc(layout);
}
```
to
```rust
const BUFFER_LAYOUT: std::alloc::Layout = unsafe {
  std::alloc::Layout::from_size_align_unchecked(0x2000, 0x1000)
};

fn foo() {
  let buffer = std::alloc::alloc(BUFFER_LAYOUT);
}
```

which (although `unsafe` is used) looks somewhat cleaner and is easier to read.

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