[mlir] Improve debug flag management in Python bindings

Expose the debug flag as a readable and assignable property of a
dedicated class instead of a write-only function. Actually test the fact
of setting the flag. Move test to a dedicated file, it has zero relation
to context_managers.py where it was added.

Arguably, it should be promoted from mlir.ir to mlir module, but we are
not re-exporting the latter and this functionality is purposefully
hidden so can stay in IR for now. Drop unnecessary export code.

Refactor C API and put Debug into a separate library, fix it to actually
set the flag to the given value.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D100757