refactor how RustIr manages ADTs (and maybe other types)

[nikomatsakis]

Currently the RustIrDatabase trait has methods that return Arc<SomeRustIrType>, but this doesn't map that well to rustc, as it must create these awkward types. We could instead have finer-grained methods. For the case of ADTs, we might have e.g.:

• num_variants(adt_id)
• num_fields(adt_id, variant_index)
• field_type(adt_id, variant_index, field_index)

This issue has been assigned to @doctorn via this comment.

[doctorn]

@rustbot claim