Auto merge of #78863 - KodrAus:feat/simd-array, r=oli-obk

Auto merge of #78863 - KodrAus:feat/simd-array, r=oli-obk

Support repr(simd) on ADTs containing a single array field

This is a squash and rebase of `@gnzlbg's` #63531

I've never actually written code in the compiler before so just fumbled my way around until it would build 😅

I imagine there'll be some work we need to do in `rustc_codegen_cranelift` too for this now, but might need some input from `@bjorn3` to know what that is.

cc `@rust-lang/project-portable-simd`

-----

This PR allows using `#[repr(simd)]` on ADTs containing a single array field:

```rust
 #[repr(simd)] struct S0([f32; 4]);
 #[repr(simd)] struct S1<const N: usize>([f32; N]);
 #[repr(simd)] struct S2<T, const N: usize>([T; N]);
```

This should allow experimenting with portable packed SIMD abstractions on nightly that make use of const generics.