Mark Parser2::empty as const

src/test.cpp

@@ -599,7 +599,7 @@ private:
   void *const data;
   Symbol stack[kMaxStackSize];
   Symbol *stackPtr = stack;
-  bool empty() { return stackPtr == stack; }
+  bool empty() const { return stackPtr == stack; }
   void pop() {
     assert(!empty());
     --stackPtr;