AST: Relax containment for Swift, PackageDescription, and Embedded domains.

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.

Author: tshortli

lib/AST/AvailabilityDomain.cpp

@@ -147,26 +147,19 @@ ModuleDecl *AvailabilityDomain::getModule() const {
 }
 
 bool AvailabilityDomain::contains(const AvailabilityDomain &other) const {
-  // FIXME: [availability] This currently implements something closer to a
-  // total ordering instead of the more flexible partial ordering that it
-  // would ideally represent. Until AvailabilityContext supports tracking
-  // multiple unavailable domains simultaneously, a stricter ordering is
-  // necessary to support source compatibility.
   switch (getKind()) {
   case Kind::Universal:
     return true;
   case Kind::SwiftLanguage:
-    return !other.isUniversal();
   case Kind::PackageDescription:
   case Kind::Embedded:
-    return !other.isUniversal() && !other.isSwiftLanguage();
+  case Kind::Custom:
+    return other == *this;
   case Kind::Platform:
     if (getPlatformKind() == other.getPlatformKind())
       return true;
     return inheritsAvailabilityFromPlatform(other.getPlatformKind(),
                                             getPlatformKind());
-  case Kind::Custom:
-    return getCustomDomain() == other.getCustomDomain();
   }
 }
 

unittests/AST/AvailabilityDomainTests.cpp

@@ -52,16 +52,10 @@ TEST_F(AvailabilityDomainLattice, Contains) {
     // The universal domain is the bottom domain and contains all others.
     EXPECT_TRUE(Universal.contains(domain));
 
-    // FIXME: [availability] The following assertions should change when
-    // AvailabilityContext can support multiple simultaneous unavailable
-    // domains.
-
-    // The Swift domain is second from the bottom.
-    EXPECT_EQ(Swift.contains(domain), !domain.isUniversal());
-
-    // Package and Embedded are both third from the bottom.
-    EXPECT_EQ(Package.contains(domain), !domain.isUniversal() && !domain.isSwiftLanguage());
-    EXPECT_EQ(Embedded.contains(domain), !domain.isUniversal() && !domain.isSwiftLanguage());
+    // These domains only contain themselves.
+    EXPECT_EQ(Swift.contains(domain), domain.isSwiftLanguage());
+    EXPECT_EQ(Package.contains(domain), domain.isPackageDescription());
+    EXPECT_EQ(Embedded.contains(domain), domain.isEmbedded());
   }
 
   // Platform kind domains form their own lattice in which app extension domains