- theoretically any single field of the union could be ?Sized, but it's easier to implement this way
- still have to allow/implement the unsize cast for unions

this supports unsized unions, where the last field may be unsized.
Theoretically, any number of fields of a union could be unsized, as
long as the unsized type is the same for all of them, but this is the
simplest way to get ManuallyDrop<T: ?Sized> working.

this now only fails if the unsized field is not the last field

Adds an extra field to the unsized union, so that there will be an
error about the union needing to only have a single field

- the old implementation had an `all_sized: bool` parameter which was
true iff the type was a struct, and allowed the last field to be
unsized in that case.
- Because of unsized unions, things are more complicated, and will
become more complicated in the future if we get multiple unsized union
fields or enum variants
- this implementation gets rid of the `all_sized` parameter, and just
checks directly in the function whether the type is a struct, enum or
union, handling each case independently

The expression `slice.len()-1` underflows when the slice is empty,
which caused an out-of-bounds array access. At least Rust panics in
that case!

I wish there was a more idiomatic way to get a sub-slice with
everything but the last element. This way is evidently pretty bug-prone

This should output a better error message if check_type_defn ever gets
called with something that’s not a struct, enum or union

- update the comments on the `U6` test case, to make it more clear what
to do when non-`Copy` unions are stabilized
- add a crucial `~` that caused a test to pass that wasn’t supposed to
- update stderr of some tests to reflect changes in the actual tests
and compiler error messages

- unsized unions require the untagged_unions feature
- prececedence is given to other error messages for the untagged_unions feature gate. For example, if there is already going to be an error about non-`Copy` unions being unstable, we do not also emit an error about unsized unions being unstable