GROOVY-10561: @NamedVariant self referential default values are not c…

[paulk-asert]

…orrectly resolved

[paulk-asert]

The PR works for the simple case of the initial value being a reference to a previously defined parameter, as in the provided example, but should really be a visitor so that it could handle expressions involving such parameters. I'll do that next.
Now also handling subexpressions.

[eric-milles]

Is this the same way Verifier handles it for normal methods?

[paulk-asert]

This has been simplified to be more like Verifier. I think there is still scope for catering for additional subexpressions but we can cover that in future changes.