Rollup merge of #137280 - RalfJung:const_swap_nonoverlapping, r=lcnr

stabilize ptr::swap_nonoverlapping in const

Closes rust-lang/rust#133668

The blocking issue mentioned there is resolved by documentation. We may in the future actually support such code, but that is blocked on rust-lang/const-eval#72 which is non-trivial to implement. Meanwhile, this completes stabilization of all `const fn` in `ptr`. :)

Here's a version of the problematic example to play around with:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=6c390452379fb593e109b8f8ee854d2a

Should be FCP'd with both `@rust-lang/libs-api`  and `@rust-lang/lang`  since  `swap_nonoverlapping` is documented to work as an "untyped" operation but due to the limitation mentioned above, that's not entirely true during const evaluation. I expect this limitation will only be hit in niche corner cases, so the benefits of having this function work most of the time outweigh the downsides of users running into this problem. (Note that unsafe code could already hit this limitation before this PR by doing cursed pointer casts, but having it hidden inside `swap_nonoverlapping` feels a bit different.)

Author: tgross35