Implement the @export(implementation) and @export(interface) attributes
to replace @_alwaysEmitIntoClient and @_neverEmitIntoClient. Provide a
warning + Fix-It to start staging out the very-new
@_neverEmitIntoClient. We'll hold off on pushing folks toward
@_alwaysEmitIntoClient for a little longer.
Don't consider implicitly exposed memory layouts when checking for
usable from inline correctness. That check applies only to memory
layouts marked as exposed explicitly. Consider the implict state only at
the general availability checking.
In non-library-evolution mode, gated behind the CheckImplementationOnly
feature flag, report references to structs marked with
`@_implementationOnly` from a fragile context. Preventing references
from inlinable functions and structs not marked `@_implementationOnly`.
