AST: Introduce AvailabilityDomain::contains().
This operation describes the partial ordering with which Availability domains
form a lattice.
As a temporary measure, a containment ordering needs to be specified for the
Swift language, Embedded, and Package Description domains. Without this
ordering, there won't be a way for AvailabilityContext to preserve the
invariant that the unavailable domain of a child context contains the
unavailable domain for the parent. However, once AvailabilityContext is
refactored to represent the status of multiple availability domains
simultaneously, the ordering of these domains relative to each other can be
relaxed.
NFC.
