[mlir] python enum bindings generator

Add an ODS (tablegen) backend to generate Python enum classes and
attribute builders for enum attributes defined in ODS. This will allow
us to keep the enum attribute definitions in sync between C++ and
Python, as opposed to handwritten enum classes in Python that may end up
using mismatching values. This also makes autogenerated bindings more
convenient even in absence of mixins.

Use this backend for the transform dialect failure propagation mode enum
attribute as demonstration.

Reviewed By: ingomueller-net

Differential Revision: https://reviews.llvm.org/D156553