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.
Consider functions in embedded mode to be fragile unless marked with
`@_neverEmitIntoClient`. Basically treating them as if they were
`@_alwaysEmitIntoClient` by default. A fragile function cannot reference
restricted imports such as `@_implementationOnly`, `internal import`,
etc.
We consider them as fragile only for type-checking, at the SIL level
they remain treated as normal functions like before. This allows the
later optimization to work as expected. We may want to likely align both
ends of the compiler once this is properly supported in sema.
This is enabled only in embedded. Fragile functions in library-evolution
are marked with attributes and are already checked. We do not need this
is non-embedded non-library-evolution as CMO handles inlining and
already takes into consideration `@_implementationOnly` imports.
Note: We'll need a similar check for memory layouts exposed to clients.
We can still see compiler crashes and miscompiles in this scenario. This
will apply to both embedded and non-library-evolution modes. That will
likely need to extend the `@_neverEmitIntoClient` attribute and may
require a staged deployment.
rdar://161365361
