Add an option to enable a copy-on-write existential implementation

lib/Frontend/CompilerInvocation.cpp

@@ -1237,6 +1237,10 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
     IRGenOpts.Sanitize = Opts.Sanitize;
   }
 
+  /// Should we use the copy-on-write implementation of opaque existentials.
+  /// FIXME: Use during bootstraping this feature. Remove later.
+  Opts.UseCOWExistentials = Args.hasArg(OPT_enable_cow_existentials);
+
   return false;
 }