Change untagged_unions to not allow union fields with drop

Change untagged_unions to not allow union fields with drop

Union fields may now never have a type with attached destructor.
This for example allows unions to use arbitrary field types only by
wrapping
them in ManuallyDrop.

The stable rule remains, that union fields must be Copy. We use the new
rule for the `untagged_union` feature.

See RFC 2514.

Note for ui tests:
We can't test move out through Box's deref-move since we can't
have a Box in a union anymore.