Rollup merge of #77547 - RalfJung:stable-union-drop, r=matthewjasper

stabilize union with 'ManuallyDrop' fields and 'impl Drop for Union'

As [discussed by @SimonSapin and @withoutboats](https://github.com/rust-lang/rust/issues/55149#issuecomment-634692020), this PR proposes to stabilize parts of the `untagged_union` feature gate:

* It will be possible to have a union with field type `ManuallyDrop<T>` for any `T`.
* While at it I propose we also stabilize `impl Drop for Union`; to my knowledge, there are no open concerns around this feature.

In the RFC discussion, we also talked about allowing `&mut T` as another non-`Copy` non-dropping type, but that felt to me like an overly specific exception so I figured we'd wait if there is actually any use for such a special case.

Some things remain unstable and still require the `untagged_union` feature gate:
* Union with fields that do not drop, are not `Copy`, and are not `ManuallyDrop<_>`. The reason to not stabilize this is to avoid semver concerns around libraries adding `Drop` implementations later. (This is already not fully semver compatible as, to my knowledge, the borrow checker will exploit the non-dropping nature of any type, but it seems prudent to avoid further increasing the amount of trouble adding an `impl Drop` can cause.)

Due to this, quite a few tests still need the `untagged_union` feature, but I think the ones where I could remove the feature flag provide good test coverage for the stable part.

Cc @rust-lang/lang

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