In the future, the `DeclContext` for a given scope will be needed as an input
in order to query for the `AvailabilityDomain` associated with an
`AvailabilitySpec`.
Represent an AvailabilityDomain as a pointer union where one member of the
union is inline storage for built-in domains with known requirements and the
other member of the union is a pointer to an externally allocated domain
definition.
NFC.
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.
