IRGen: Emit runtime initialization into top_level_code for playgrounds.

To help with the playground's transition to top level code, hand the -playground frontend flag down to IRGenOptions, so that IRGen knows to emit runtime initializer code for classes and categories into top_level_code rather than an attributed function.

Swift SVN r18479

lib/Frontend/CompilerInvocation.cpp

@@ -872,6 +872,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
   Opts.OutputFilename = FrontendOpts.OutputFilename;
   Opts.ModuleName = FrontendOpts.ModuleName;
 
+  if (Args.hasArg(OPT_playground))
+    Opts.Playground = true;
+  
   return false;
 }