SuppAssocTypes: add deprecation warning of legacy version
We need to start moving people over to using the version of
suppressed associated types that has the officially accepted
behavior according to SE-503, and that is via
`SuppressedAssociatedTypesWithDefaults`.
To allow for time to migrate to the new version, I'm introducing
the deprecation message as a warning for now.
This set of changes also moves most of the test suite over to using
the officially accepted version.
rdar://172231663
