swift

06443b96 - AST: Refactor AvailabilityDomain to prepare for external definitions.

AST: Refactor AvailabilityDomain to prepare for external definitions. 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.