Rollup merge of #147708 - clarfonthey:const-drop, r=oli-obk

const `mem::drop`

I'm putting this under the `const Destruct` feature flag since it doesn't really feel relevant to put it elsewhere… it's just an empty function body, so, it doesn't have any particular weirdness attached to it (unlike `drop_in_place`, for example).

r? wg-const-eval

Author: matthiaskrgr