* Translate the following versions to 26.
* watchOS 12 -> 26
* visionOS 3 -> 26
* macos 16 -> 26
* iOS 19 -> 26
* tvOS 19 -> 26
* Emit diagnostics, but allow conversion when clients attempt to use
invalid gaps in OS versioning in availability.
* For target-triples, only allow "valid" versions for implicit
conversions.