ModuleInterface: Handle custom availability domains for synthesized conformances.

When gathering availability attributes to print in a .swiftinterface for
synthesized conformances to inherited protocols, don't assume availabilty
attributes always have a platform kind.

Resolves rdar://170361240.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -456,10 +456,9 @@ class InheritedProtocolCollector {
       for (auto nextAttr : D->getSemanticAvailableAttrs()) {
         // FIXME: This is just approximating the effects of nested availability
         // attributes for the same platform; formally they'd need to be merged.
-        // FIXME: [availability] This should compare availability domains.
         bool alreadyHasMoreSpecificAttrForThisPlatform = llvm::any_of(
             *cache, [nextAttr](SemanticAvailableAttr existingAttr) {
-              return existingAttr.getPlatform() == nextAttr.getPlatform();
+              return existingAttr.getDomain() == nextAttr.getDomain();
             });
         if (alreadyHasMoreSpecificAttrForThisPlatform)
           continue;
@@ -695,7 +694,7 @@ public:
     if (!printOptions.shouldPrint(nominal))
       return;
 
-    /// is this nominal specifically an 'actor' or 'distributed actor'?
+    // Is this nominal specifically an 'actor' or 'distributed actor'?
     bool anyActorClass = false;
     if (auto klass = dyn_cast<ClassDecl>(nominal)) {
       anyActorClass = klass->isAnyActor();