[mlir] Remove the need to define `kindof` on attribute and type classes.

This revision refactors the default definition of the attribute and type `classof` methods to use the TypeID of the concrete class instead of invoking the `kindof` method. The TypeID is already used as part of uniquing, and this allows for removing the need for users to define any of the type casting utilities themselves.

Differential Revision: https://reviews.llvm.org/D85356