Availability version remapping currently only applies to code built for
visionOS. We plan to introduce more platform kinds and standalone availability
domains that will require version remapping, though, so it's time to
rearchitect and simplify the code to make it easier to generalize.
`AvailabilityDomain` is now responsible for version remapping and much of the
previously duplicated utilities have been consolidated.
When compiling for visionOS, iOS availability attributes are remapped into the
visionOS availability domain automatically. While the version remapping was
being performed correctly, there was a regression that caused the platform name
to be printed incorrectly in many diagnostics. Whenever an iOS version is
remapped to a visionOS version, availability diagnostics will now present
those versions as visionOS versions instead of iOS versions.
Resolves rdar://146293165.
Implements SE-0460 -- the non-underscored version of @specialized.
It allows to specify "internal" (not abi affecting) specializations.
rdar://150033316
Put AvailabilityRange into its own header with very few dependencies so that it
can be included freely in other headers that need to use it as a complete type.
NFC.
