[mlir] Expand default arg builders to wrapped attributes.

Previously only unwrapped values were considered for default values in
builders, expand to Attributes given the change to populate defaults. To avoid
overlap between wrapped and unwrapped, the starting index is incremented
depending on if the smallest starting index for default valued args can be
materialized in unwrapped form or not.

Given the above limitation this is pretty small change.

`optional` is unfortunately meant to also represent conditionally set which
means we cannot allow nullptr for all Attributes marked optional at the moment.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D140705