ABI checker: exclude unavailable decls from ABI descriptors

Framework authors usually have different schemes for different deployment
targets. We should exclude platform-unavailable ABIs from the Json file so
developers will only be warned of the breakages that are relevant to the current
scheme.

rdar://54273296

tools/swift-api-digester/ModuleAnalyzerNodes.cpp

@@ -1570,9 +1570,9 @@ SDKContext::shouldIgnore(Decl *D, const Decl* Parent) const {
   } else {
     if (D->isPrivateStdlibDecl(false))
       return true;
-    if (AvailableAttr::isUnavailable(D))
-      return true;
   }
+  if (AvailableAttr::isUnavailable(D))
+     return true;
   if (auto VD = dyn_cast<ValueDecl>(D)) {
     switch (getAccessLevel(VD)) {
     case AccessLevel::Internal: