[augmentations] Ensure correct behavior in unreachable_from_main

[srawlins]

Child of #59399

unreachable_from_main walks all compilation units (parts) to determine an element's reachability (including public elements).

If a macro application augments a class with a constructor and/or a method, those need to be walked as well. For example, if a class has a public field which looks unused, but it used by a method found in an augmentation, that should be considered (unless we don't report unreachable instance fields/methods?).