Declarations that are unavailable at runtime because of an `@available`
attribute referencing a custom domain that was imported `@_spiOnly` should be
hidden from public swiftinterface files in `-library-level=api` modules. For
remaining declarations that do get printed in the public swiftinterface, skip
printing any `@available` attribute that refers to the domains from those
`@_spiOnly` dependencies. This allows API developers to control declaration
visibility using availability defined by another module.
Resolves rdar://156512028.
