Auto merge of #46123 - Gankro:c-repr, r=eddyb

Implement the special repr(C)-non-clike-enum layout

This is the second half of https://github.com/rust-lang/rfcs/pull/2195

which specifies that

```rust
#[repr(C, u8)]
#[derive(Copy, Clone, Eq, PartialEq, Debug)]
enum MyEnum {
    A(u32),                 // Single primitive value
    B { x: u8, y: i16 },    // Composite, and the offset of `y` depends on tag being internal
    C,                      // Empty
    D(Option<u32>),         // Contains an enum
    E(Duration),            // Contains a struct
}
```

Has the same layout as

```rust
#[repr(C)]
struct MyEnumRepr {
    tag: MyEnumTag,
    payload: MyEnumPayload,
}

#[repr(C)]
#[allow(non_snake_case)]
union MyEnumPayload {
    A: MyEnumVariantA,
    B: MyEnumVariantB,
    D: MyEnumVariantD,
    E: MyEnumVariantE,
}

#[repr(u8)] #[derive(Copy, Clone)] enum MyEnumTag { A, B, C, D, E }
#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantA(u32);
#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantB {x: u8, y: i16 }
#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantD(Option<u32>);
#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantE(Duration);

```

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