Add adt_def to struct and enum metadata

[jberthold]

These adt_def of struct and enum metadata are required for a reverse mapping adt_def -> Ty in the semantics.
The AggregateKind only carries the adt_def but we have to look up the discriminant information (and potentially other metadata later) from a Ty key.

Note that the adt_def is unstable, it changes with every compiler run, so we delete the field in the golden tests.