Auto merge of #152605 - scottmcm:box-drop-alignment, r=Mark-Simulacrum

Pass alignments through the shim as `Alignment` (not `usize`)

We're using `Layout` on both sides, so might as well skip the transmutes back and forth to `usize`.

The mir-opt test shows that doing so allows simplifying the boxed-slice drop slightly, for example.

Author: bors