swift

78f50ba7 - AST: Refactor ConditionallyAvailableSubstitutions to use AvailabilityQuery.

AST: Refactor ConditionallyAvailableSubstitutions to use AvailabilityQuery. Conditionally available opaque return types should support availability conditions that are evaluated in any availability domain. Update `ConditionallyAvailableSubstitutions` to model its conditions with `AvailabilityQuery` instead of assuming that conditions are always a single version query for the current platform.