Auto merge of #6046 - rail-rain:change_criteria_non_copy_const, r=flip1995

Change the criteria of `interior_mutable_const`

This implements my suggestion [here](https://github.com/rust-lang/rust-clippy/issues/5050#issuecomment-680310889), and so hopefully fixes #5050.

* stop linting associated types and generic type parameters
* start linting ones in trait impls
  whose corresponding definitions in the traits are generic
* remove the `is_copy` check
  as presumably the only purpose of it is to allow
  generics with `Copy` bounds as `Freeze` is internal
  and generics are no longer linted
* remove the term 'copy' from the tests
  as being `Copy` no longer have meaning

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changelog: Change the criteria of `declare_interior_mutable_const` and `borrow_interior_mutable_const` to narrow the lints to only lint things that defenitly is a interior mutable type, not potentially.