AST: Weak-link declarations in extensions of weak-linked types
This is a workaround to fix weak linking of frameworks that define
types with availability, but then define extensions of those types
without availability.
This can come up if the framework itself is built with a newer
deployment target than the client that uses the framework. Since the
type checker only enforces that an extension has availability if
the extension is less available than the deployment target, we were
failing to weak link the members of the extension in this case.
This is not a perfect fix; ideally such frameworks should be built
with -require-explicit-availability, and all reported warnings
fixed by adding explicit availability.
However, it allows clients to weak link when using existing
swiftinterface files that have already shipped in the mean time,
and it should not cause any problems once the frameworks are properly
annotated in the future.
Fixes <rdar://problem/58490723>.
