Revisit isEmpty methods

[apple]

Revisit isEmpty methods

[jcanizales]

<Standard reminder that adding stuff later if there's demand is always possible, but removing isn't.>

[tbkka]

I think the generated isEmpty is only still being used in one case: the equality test on messages uses it to ensure that a message with no allocated storage class is considered equal to one where the storage class is allocated but "empty."

An alternative approach would be to always allocate the backing storage object. Then this case never arises. Empty objects (allocated but with no fields set) should be uncommon, so I don't see a big drawback. It would also simplify the uniqueStorage() implementation.