IRGen: Emit reflection metadata for certain builtin types when -enable-reflection-builtins flag is passed in

These types are not directly referenced as fields of aggregate types,
but are needed for reflection type lowering.

Also, use a SetVector to collect referenced builtin types, instead of
a SmallPtrSet, to ensure compiler output is deterministic.

lib/Frontend/CompilerInvocation.cpp

@@ -1282,6 +1282,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     if (Args.hasArg(OPT_enable_reflection_names)) {
       Opts.EnableReflectionNames = true;
     }
+    if (Args.hasArg(OPT_enable_reflection_builtins)) {
+      Opts.EnableReflectionBuiltins = true;
+    }
   }
 
   return false;