We have `BackendKinds.IrBackend` for IR emitted by the frontend.
We need a different backend kind for IR deserialized from klibs.
We cannot use `BackendKinds.IrBackend` for that purpose, because we
cannot have two different `IrBackendInput` subclasses with the same
kind.
We need a different `IrBackendInput` subclass for deserialized IR
because the `IrBackendInput` for frontend-emitted IR contains some
properties like `sourceFiles` which don't make sense for
deserialized IR.
Since we now have two backend kinds for IR artifacts, we need to make
certain functions and classes in the test infrastructure generic
over backend kinds.
Note: the JsIrDeserializedFromKlibBackendInput class is not used
anywhere yet. We will use it in the coming commits.
Sergej Jaskiewicz
2727163a63