[mlir] Add support for interface inheritance

This allows for interfaces to define a set of "base classes",
which are interfaces whose methods/extra class decls/etc.
should be inherited by the derived interface. This more
easily enables combining interfaces and their dependencies,
without lots of awkard casting. Additional implicit conversion
operators also greatly simplify the conversion process.

One other aspect of this "inheritance" is that we also implicitly
add the base interfaces to the attr/op/type. The user can still
add them manually if desired, but this should help remove some
of the boiler plate when an interface has dependencies.

See https://discourse.llvm.org/t/interface-inheritance-and-dependencies-interface-method-visibility-interface-composition

Differential Revision: https://reviews.llvm.org/D140198