Rollup merge of #76807 - ecstatic-morse:const-checking-staged-api, r=oli-obk

Rollup merge of #76807 - ecstatic-morse:const-checking-staged-api, r=oli-obk

Use const-checking to forbid use of unstable features in const-stable functions

First step towards #76618.

Currently this code isn't ever hit because `qualify_min_const_fn` runs first and catches pretty much everything. One exception is `const_precise_live_drops`, which does not use the newly added code since it runs as part of a separate pass.

Also contains some unrelated refactoring, which is split into separate commits.

r? @oli-obk