Refactor Attribute and Type to use 'classof' instead of 'kindof'...

    Refactor Attribute and Type to use 'classof' instead of 'kindof' internally. If a 'classof' method is not defined, a default implementation will invoke 'kindof' on a derived type. This allows for defining supplementary Attribute/Type classes that expose additional functionality, but do not have a specific kind value. An example of this can be seen in the 'Constant(Float|Index|Int)Ops' that derive from 'ConstantOp'.

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PiperOrigin-RevId: 248724093