Add comments to PrintActions1 and Driver::PrintActions. NFC.

lib/Driver/Driver.cpp

if (C.getArgs().hasArg(options::OPT_print_multi_os_directory)) {
     // "lib/../lib32" as appropriate for the toolchain. For now, print
     // nothing because it's not supported yet.
     return false;
   }
 
   return true;
 }
 
+// Display an action graph human-readably.  Action A is the "sink" node
+// and latest-occuring action. Traversal is in pre-order, visiting the
+// inputs to each action before printing the action itself.
 static unsigned PrintActions1(const Compilation &C, Action *A,
                               std::map<Action*, unsigned> &Ids) {
-  if (Ids.count(A))
+  if (Ids.count(A)) // A was already visited.
     return Ids[A];
 
   std::string str;
   llvm::raw_string_ostream os(str);
 
   os << Action::getClassName(A->getKind()) << ", ";
   if (InputAction *IA = dyn_cast<InputAction>(A)) {
     os << "\"" << IA->getInputArg().getValue() << "\"";
static unsigned PrintActions1(const Compilation &C, Action *A,
   unsigned Id = Ids.size();
   Ids[A] = Id;
   llvm::errs() << Id << ": " << os.str() << ", "
                << types::getTypeName(A->getType()) << "\n";
 
   return Id;
 }
 
+// Print the action graphs in a compilation C.
+// For example "clang -c file1.c file2.c" is composed of two subgraphs.
 void Driver::PrintActions(const Compilation &C) const {
   std::map<Action*, unsigned> Ids;
   for (ActionList::const_iterator it = C.getActions().begin(),
          ie = C.getActions().end(); it != ie; ++it)
     PrintActions1(C, *it, Ids);
 }
 
 /// \brief Check whether the given input tree contains any compilation or