Use llvm::ListSeparator for simpler separators (#671)

I was looking for something like this, I think it's a straightforward simplification for code. Internally, it handles skipping the separator on the first print.

See the bottom of: https://llvm.org/doxygen/StringExtras_8h_source.html

executable_semantics/interpreter/action.cpp

@@ -13,6 +13,7 @@
 #include "executable_semantics/ast/expression.h"
 #include "executable_semantics/ast/function_definition.h"
 #include "executable_semantics/interpreter/stack.h"
+#include "llvm/ADT/StringExtras.h"
 
 namespace Carbon {
 
@@ -74,24 +75,21 @@ void Action::Print(llvm::raw_ostream& out) const {
   out << "<" << pos << ">";
   if (results.size() > 0) {
     out << "(";
+    llvm::ListSeparator sep;
     for (auto& result : results) {
+      out << sep;
       if (result) {
         out << *result;
       }
-      out << ",";
     }
     out << ")";
   }
 }
 
 void Action::PrintList(const Stack<Action*>& ls, llvm::raw_ostream& out) {
-  auto it = ls.begin();
-  while (it != ls.end()) {
-    out << **it;
-    ++it;
-    if (it != ls.end()) {
-      out << " :: ";
-    }
+  llvm::ListSeparator sep(" :: ");
+  for (const auto& action : ls) {
+    out << sep << *action;
   }
 }