This restriction came from wanting to make resilient and non-resilient
code follow the same rules whenever possible, but after thinking about
it a bit more we realized there was no reason why you /wouldn't/ just
mark your structs @_fixed_layout in non-resilient libraries anyway.
Since that (currently?) doesn't affect what you can do with the struct
across module boundaries, and since the layout of the struct is
available anyway in a non-resilient library, there's no real downside,
which means it's a meaningless restriction.
The same logic doesn't /quite/ apply to classes, since classes are
normally much more flexible than structs. (For example, you could add
a stored property to a class without recompiling clients, as long as
no initializers are inlined.) But it's close enough that we don't want
to put in the restriction at this time.
All of this is about attributes that haven't been finalized yet anyway
(hence the leading underscore), but it's still useful information.
rdar://problem/37408668
