make accesses to fields of packed structs unsafe

To handle packed structs with destructors (which you'll think are a rare
case, but the `#[repr(packed)] struct Packed<T>(T);` pattern is
ever-popular, which requires handling packed structs with destructors to
avoid monomorphization-time errors), drops of subfields of packed
structs should drop a local move of the field instead of the original
one.

cc #27060 - this should deal with that issue after codegen of drop glue
is updated.

The new errors need to be changed to future-compatibility warnings, but
I'll rather do a crater run first with them as errors to assess the
impact.

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