In implicit contexts that are universally unavailable, allow writable key paths
to be formed to properties with setters that are also marked as universally
unavailable. This fixes a regression from the previous commit where the code
synthesized for `@Observable` properties in universally unavailable classes was
rejected by the availability checker.
Previously, only conditional unavailability was considered, but setters that
are always unavailable must also make a keypath read-only.
Resolves rdar://155832511.
