[mlir] Make `classof` substitution in interface use an instance (#65492)

The substitution supported by `extraClassOf` is currently limited to
only the base instance, i.e. `Operation*`, `Type` or `Attribute`, which
limits the kind of checks you can perform in the `classof`
implementation.

Since prior to the user code, the interface concept is fetched, we can
use it to construct an instance of the interface, allowing use of its
methods in the `classof` check.

Since an instance of the interface allows access to the base class
methods through the `->` operator, I've gone ahead and replaced the
substitution of `$_op/$_type/$_attr` with an interface instance. This is
also consistent with `extraSharedClassDeclaration` and other methods
created in the interface class which do the same.