Rollup merge of #89509 - jhpratt:stabilize-const_unreachable_unchecked, r=oli-obk

Rollup merge of #89509 - jhpratt:stabilize-const_unreachable_unchecked, r=oli-obk

Stabilize `unreachable_unchecked` as `const fn`

Closes #53188

This PR stabilizes `core::hint::unreachable_unchecked` as `const fn`. MIRI is able to detect when this method is called. Stabilization was delayed until `const_panic` was stabilized so as to avoid users calling this method in its place (thus resulting in runtime UB). With #89508, that is no longer an issue.

````@rustbot```` label +A-const-eval +A-const-fn +T-lang +S-blocked

(not sure why it's T-lang, but that's what the tracking issue is)