Rollup merge of rust-lang#147743 - 21aslade:packed-diagnostic, r=RalfJung

Show packed field alignment in mir_transform_unaligned_packed_ref

Fixes rust-lang#147528

I left the expected padding for the field out of the error message so the message would be the same on all platforms. It also isn't always possible to know the expected alignment, so this makes the message simpler.

Author: Zalathar