Add support for referencing "forward-declared" attributes

[math-fehr]

We would like to write something like:

@irdl_attr_definition
class ChocoListTypeData:
    name = "choco.list_type_data"
    # The following is a reference to itself
    type = AttributeDef(AnyOf([ChocoTypeData, ChocoListTypeData]))

[K-W-Li]

If I remember correctly, the following should work:

@irdl_attr_definition
class ChocoListTypeData:
    name = "choco.list_type_data"
    # The following is a reference to itself
    type = AttributeDef(AnyOf([ChocoTypeData, "ChocoListTypeData"]))

But it seems that AttributeDef(AnyOf([ChocoTypeData, ChocoListTypeData])) is not normal python type?

[math-fehr]

Note that it is breaking with the new way to define attributes:

@irdl_attr_definition
class RecursiveAttr(ParametrizedAttribute):
    """An attribute referring itself."""
    name = "test.recursive_attr"

    x: ParameterDef[RecursiveAttr | BoolData]

This is because when we introspect RecursiveAttr, RecursiveAttr is not yet defined, and thus get_type_hints will fail.

[superlopuh]

Does it work with from __future__ import annotations ?

[math-fehr]

Nope, and actually, this is a more general problem than just "recursive attributes". It will fail for any forward-referenced attribute. The problem is that when introspecting the code, forward referenced attributes are not defined yet, so the introspection fails.

One solution could be to delay the verification code creation to "runtime", after all classes have been parsed. This should be doable I think.