Rollup merge of #121894 - RalfJung:const_eval_select, r=oli-obk

const_eval_select: make it safe but be careful with what we expose on stable for now

As this is all still nightly-only I think `````@rust-lang/wg-const-eval````` can do that without involving t-lang.

r? `````@oli-obk`````
Cc `````@Nilstrieb````` -- the updated version of your RFC would basically say that we can remove these comments about not making behavior differences visible in stable `const fn`

Author: matthiaskrgr