Fix a FP in `missing_const_for_fn`
where a function that calls a standard library function whose constness
is unstable is considered as being able to be a const function. Fixes#5995.
The core change is the move from `rustc_mir::const_eval::is_min_const_fn` to `rustc_mir::const_eval::is_const_fn`. I'm not clear about the difference in their purpose between them so I'm not sure if it's acceptable to call `qualify_min_const_fn::is_min_const_fn` this way now.
---
changelog: `missing_const_for_fn`: No longer lints when an unstably const function is called
19740d9334