[Attributor] Introduce helpers to judge AAs prior to creation

This is a partial cleanup to centralize the initialization and update
decisions for AAs. Lifting the burdon and boilerplate on users and
making it harder to accidentally perform unsound deductions.

The two static helpers show how we can lift the decisions to generate an
AA into the Attributor, avoiding trivial AAs that just cost us compile
time and maintenance code (to check for pre-conditions).