Replace the `here` part of the generic exportability diagnostic for
variables with: `in a property declaration marked public or in a
'@frozen' or '@usableFromInline' context`.
The full diagnostic now looks like:
```
error: cannot use struct 'ImportedType' in a property declaration marked
public or in a '@frozen' or '@usableFromInline' context;
'HiddenDependency' has been imported as implementation-only
```
This should be improved further to support implicitly exported memory
layouts in non-library-evolution and embedded.
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.
