Auto merge of #116015 - EvanMerlock:master, r=oli-obk

const_eval: allow function pointer signatures containing &mut T in const contexts

potentially fixes #114994

We utilize a `TypeVisitor` here in order to more easily handle control flow.
- In the event the typekind the Visitor sees is a function pointer, we skip over it
- However, otherwise we do one of two things:
   - If we find a mutable reference, check it, then continue visiting types
   - If we find any other type, continue visiting types

This means we will check if the function pointer _itself_ is mutable, but not if any of the types _within_ are.

Author: bors