Investigate automatically running HHAST in repo-authoritative mode

[fredemmott]

Linters that use reflection (e.g. 'must use __Override attribute') would need to be disabled; once we support cross-file linters fully, we'd presumably have a database of facts about various definitions that could be used to refactor them.

[lexidor]

HHClientLinter contains an equivalent lint. It finds more cases than MustUseOverrideLinter. Disabling MustUseOverrideLinter is not as great a loss as it used to be.

Refs #432 😞