Move AvailabilityAttr::PlatformKind into its own file

Swift SVN r21728

lib/Serialization/Serialization.cpp

@@ -1398,7 +1398,7 @@ void Serializer::writeDeclAttribute(const DeclAttribute *DA) {
         LIST_VER_TUPLE_PIECES(Introduced),
         LIST_VER_TUPLE_PIECES(Deprecated),
         LIST_VER_TUPLE_PIECES(Obsoleted),
-        theAttr->Platform,
+        static_cast<unsigned>(theAttr->Platform),
         theAttr->Message.size(),
         theAttr->Rename.size(),
         blob);