[ClangImporter] Start importing Objective-C references using UncheckedOptional.

This is hidden behind the frontend flag -enable-objc-optional. Use -Xfrontend
when invoking the Swift driver.

Part of <rdar://problem/15189135>

Swift SVN r14332

lib/Frontend/CompilerInvocation.cpp

@@ -534,6 +534,8 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts, ArgList &Args,
     Opts.ExtraArgs.push_back(A->getValue());
   }
 
+  Opts.EnableOptional = Args.hasArg(OPT_enable_objc_optional);
+
   return false;
 }