Add a staging flag -enable-experimental-deserialization-recovery.

No promises that this is stable yet!

lib/Frontend/CompilerInvocation.cpp

@@ -874,6 +874,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableClassResilience |=
     Args.hasArg(OPT_enable_class_resilience);
 
+  Opts.EnableDeserializationRecovery |=
+    Args.hasArg(OPT_enable_experimental_deserialization_recovery);
+
   Opts.DisableAvailabilityChecking |=
       Args.hasArg(OPT_disable_availability_checking);