Let's point out the associated type with a separate note, so
that we can diagnose other availability failures, such as
deprecation and OS version availability.
Even if a nominal type does not have its own generic parameters,
we need to visit its conformances, because it might be defined
in a constrained extension or have a 'where' clause of its own.
Availability checking already knows how to check exportability for
types and conformances referenced from inlinable function signatures
and bodies.
Let's generalize this to work on non-inlinable function signatures
as well, allowing us to get rid of the ExportabilityChecker altogether.
