Auto merge of #36016 - petrochenkov:union, r=nikomatsakis

Implement untagged unions (RFC 1444)

cc https://github.com/rust-lang/rust/issues/32836

Notes:
- The RFC doesn't talk about `#[packed]` unions, this implementation supports them, packing changes union's alignment to 1 and removes trailing padding.
- The RFC doesn't talk about dynamically sized unions, this implementation doesn't support them and rejects them during wf-checking (similarly, dynamically sized enums are not supported as well).
- The lint for drop fields in unions can't work precisely before monomorphization, so it works pessimistically - non-`Copy` generic fields are reported, types not implementing `Drop` directly, but having non-trivial drop code are reported.

    ```
    struct S(String); // Doesn't implement `Drop`
    union U<T> {
        a: S, // Reported
        b: T, // Reported
    }
    ```

- https://github.com/rust-lang/rust/pull/35764 was indeed helpful and landed timely, I didn't have to implement internal drop flags for unions.
- Unions are not permitted in constant patterns, because matching on union fields is unsafe, I didn't want unsafety checker to dig into all constants to uncover this possible unsafety.
- The RFC doesn't talk about `#[derive]`, generally trait impls cannot be derived for unions, but some of them can. I implemented only `#[derive(Copy)]` so far. In theory shallow `#[derive(Clone)]` can be derived as well if all union fields are `Copy`, I left it for later though, it requires changing how `Clone` impls are generated.
- Moving union fields is implemented as per https://github.com/rust-lang/rust/issues/32836#issuecomment-242511491.
- Testing strategy: union specific behavior is tested, sometimes very basically (e.g. debuginfo), behavior common for all ADTs (e.g. something like coherence
checks) is not generally tested.

r? @eddyb

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