[es6] Tail calls support.

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index c6a5444d8ac94fa2d29cefa2a5350b6033a27f51..0e24d5933269ca0f00cb212e03173dba50da1c45 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -754,6 +754,21 @@ inline std::ostream& operator<<(std::ostream& os, ConvertReceiverMode mode) {
   return os;
 }
 
+// Defines whether tail call optimization is allowed.
+enum class TailCallMode : unsigned { kAllow, kDisallow };
+
+inline size_t hash_value(TailCallMode mode) { return bit_cast<unsigned>(mode); }
+
+inline std::ostream& operator<<(std::ostream& os, TailCallMode mode) {
+  switch (mode) {
+    case TailCallMode::kAllow:
+      return os << "ALLOW_TAIL_CALLS";
+    case TailCallMode::kDisallow:
+      return os << "DISALLOW_TAIL_CALLS";
+  }
+  UNREACHABLE();
+  return os;
+}
 
 // Used to specify if a macro instruction must perform a smi check on tagged
 // values.