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.
