Get the attribute working for more link entity kinds, which addresses
all the FIXME:s in the original test case.
Now the protocol resilience tests can be updated to use @_weakLinked
for all newly-added protocol requirements and default implementations.
This allows the tests to pass in the backward deployment test scenario
as well.
Eventually this will be based on availability instead of a special
attribute.
This completes <rdar://problem/29888071>.
This is mostly intended to be used for testing at this point; in the
long run, we want to be using availability information to decide
whether to weak-link something or not. You'll notice a bunch of FIXMEs
in the test case that we may not need now, but will probably need to
handle in the future.
Groundwork for doing backward-deployment execution tests.
