Rollup merge of #122647 - RalfJung:box-to-raw-retag, r=oli-obk

add_retag: ensure box-to-raw-ptr casts are preserved for Miri

In rust-lang/rust#122233 I added `retag_box_to_raw` not realizing that we can already do `addr_of_mut!(*bx)` to turn a box into a raw pointer without an intermediate reference. We just need to ensure this information is preserved past the ElaborateBoxDerefs pass.

r? ``@oli-obk``

Author: matthiaskrgr