Add a comment explaining why a function exists

llvm-svn: 365212

clang/lib/Driver/Driver.cpp

@@ -966,6 +966,9 @@ Compilation *Driver::BuildCompilation(ArrayRef<const char *> ArgList) {
   InputArgList Args = std::move(HasConfigFile ? std::move(*CfgOptions)
                                               : std::move(*CLOptions));
 
+  // The args for config files or /clang: flags belong to different InputArgList
+  // objects than Args. This copies an Arg from one of those other InputArgLists
+  // to the ownership of Args.
   auto appendOneArg = [&Args](const Arg *Opt, const Arg *BaseArg) {
       unsigned Index = Args.MakeIndex(Opt->getSpelling());
       Arg *Copy = new llvm::opt::Arg(Opt->getOption(), Opt->getSpelling(),