Instead of using a visitor in typeck, we just check, whenever lowering a
use of a param, whether the parent item is an MCG anon const during hir
ty lowering (and instantiate_value_path). If so, we report an error
since MCG anon consts should never be able to use generics. All other
kinds of anon consts are at least syntactically allowed to use generic
params.
We use a `TypeFolder` to accomplish this; this way, we look at the
fully explicit semantic representation of the type/const/whatever and
don't miss subtle cases like `Self` type aliases.
