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`
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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.
760430e6fd