[clang][cli] Fix round-trip of OPT_plugin_arg

[john.brawn]

Summary

The test Frontend/plugin-delayed-template.cpp is failing when asserts are enabled because it hits an assertion in denormalizeStringImpl when trying to round-trip OPT_plugin_arg. Fix this by adjusting how the option is handled, as the first part is joined to -plugin-arg and the second is separate.

[jansvoboda11]

LGTM, thanks!

clang/lib/Frontend/CompilerInvocation.cpp

 static void denormalizeStringImpl(SmallVectorImpl<const char *> &Args,
                                   const char *Spelling,
                                   CompilerInvocation::StringAllocator SA,
                                   Option::OptionClass OptClass, unsigned,
                                   const Twine &Value) {
   switch (OptClass) {
   case Option::SeparateClass:
   case Option::JoinedOrSeparateClass:
+  case Option::JoinedAndSeparateClass:
     Args.push_back(Spelling);
     Args.push_back(SA(Value));
     break;
   case Option::JoinedClass:
   case Option::CommaJoinedClass:
     Args.push_back(SA(Twine(Spelling) + Value));
     break;
   default:
#undef FRONTEND_OPTION_WITH_MARSHALLING
   if (Opts.ProgramAction == frontend::FixIt && !Opts.FixItSuffix.empty()) {
     GenerateProgramAction = [&]() {
       GenerateArg(Args, OPT_fixit_EQ, Opts.FixItSuffix, SA);
     };
   }
 
   GenerateProgramAction();
 
-  for (const auto &PluginArgs : Opts.PluginArgs)
+  for (const auto &PluginArgs : Opts.PluginArgs) {
+    Option Opt = getDriverOptTable().getOption(OPT_plugin_arg);
+    const char *Spelling =
+        SA(Opt.getPrefix() + Opt.getName() + PluginArgs.first);
     for (const auto &PluginArg : PluginArgs.second)
-      GenerateArg(Args, OPT_plugin_arg, PluginArgs.first + PluginArg, SA);
+      denormalizeString(Args, Spelling, SA, Opt.getKind(), 0, PluginArg);
+  }
 
   for (const auto &Ext : Opts.ModuleFileExtensions)
     if (auto *TestExt = dyn_cast_or_null<TestModuleFileExtension>(Ext.get()))
       GenerateArg(Args, OPT_ftest_module_file_extension_EQ, TestExt->str(), SA);
 
   if (!Opts.CodeCompletionAt.FileName.empty())
     GenerateArg(Args, OPT_code_completion_at, Opts.CodeCompletionAt.ToString(),
                 SA);