https://github.com/swiftlang/swift/pull/79807 caused a regression in which
`AvailabilityContext` stopped tracking the available version range for the
active platform domain for certain platforms. Fix this by reverting to checking
`AvailabilityDomain::isActive()` to determine when a given platform
`AvailabilityDomain` represents the target platform. The compiler's existing
mapping from target triple to platform domain is incomplete and it's not clear
to me whether fixing that could cause other regressions.
Resolves rdar://147413616.
Now that `AvailabilityContext` supports multiple unavailable domains, it's no
longer necessary to have a total ordering amongst the Swift,
PackageDescription, and Embedded availability domains.
One to get the active domain for the compilation target and another to get the
ABI compatibility domain for a given domain. The ABI compatibility domain will
be needed for queries that compute whether an unavailable declaration is still
reachable at runtime.
NFC.
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.
