`anyAppleOS` represents a meta-platform for availability checks that can be
used to check availability across all of Apple's operating systems. It
supports versions 26.0 and up since version 26.0 is the first OS version number
that is aligned accross macOS, iOS, watchOS, tvOS, and visionOS.
Apple platform-specific availability specification take precedence over
`anyAppleOS` availability specifications when specified simultaneously.
Resolves rdar://153834380.
This change just stages in a few new platform kinds, without fully adding
support for them yet.
- The `Swift` platform represents availability of the Swift runtime across all
platforms that support an ABI stable Swift runtime (see the pitch at
https://forums.swift.org/t/pitch-swift-runtime-availability/82742).
- The `anyAppleOS` platform is an experimental platform that represents all of
Apple's operating systems. This is intended to simplify writing availability
for Apple's platforms by taking advantage of the new unified OS versioning
system announced at WWDC 2025.
- The `DriverKit` platform corresponds to Apple DriverKit which is already
supported by LLVM.
