Auto merge of #137412 - scottmcm:redo-swap, r=cuviper

Ensure `swap_nonoverlapping` is really always untyped

This replaces #134954, which was arguably overcomplicated.

## Fixes #134713

Actually using the type passed to `ptr::swap_nonoverlapping` for anything other than its size + align turns out to not work, so this goes back to always erasing the types down to just bytes.

(Except in `const`, which keeps doing the same thing as before to preserve `@RalfJung's` fix from #134689)

## Fixes #134946

I'd previously moved the swapping to use auto-vectorization *on bytes*, but someone pointed out on Discord that the tail loop handling from that left a whole bunch of byte-by-byte swapping around.  This goes back to manual tail handling to avoid that, then still triggers auto-vectorization on pointer-width values.  (So you'll see `<4 x i64>` on `x86-64-v3` for example.)

Author: bors