enforcing 'UNSPECIFIED' for 0 in enums that are bitmasks feels wrong

[wijnandatgoogle]

Any enum value of 0 must have a name that ends with _UNSPECIFIED at the moment. That makes sense for regular enums that relate to something you can pick from a list. But when the enum is a bitmask, the value 0 is a valid and useful number. If there needs to be a check on naming conventions there too, I would suggest enforcing that it ends with _NONE. None makes sense to me because none of the flags in the bitmask are set.