Add the @NSKeyedArchiveLegacy attribute.

This attribute allows one to provide the "legacy" name of a class for
the purposes of archival (via NSCoding). At the moment, it is only
useful for suppressing the warnings/errors about classes with unstable
archiving names.

lib/Serialization/Serialization.cpp

@@ -1948,6 +1948,7 @@ void Serializer::writeDeclAttribute(const DeclAttribute *DA) {
   case DAK_SynthesizedProtocol:
   case DAK_Count:
   case DAK_Implements:
+  case DAK_NSKeyedArchiveLegacy:
     llvm_unreachable("cannot serialize attribute");
     return;