[llvm][utils] Fix innocuous off by one in lldb formatters

num_children is "last_index" + 1, thus
    num_children + 1 = "last_index" + 2

this worked anyway because the index of `$$dereference$$` would work as long as
it was past the last index.

llvm/utils/lldbDataFormatters.py

@@ -132,7 +132,7 @@ def num_children(self):
 
     def get_child_index(self, name):
         if name == '$$dereference$$':
-            return self.valobj.num_children + 1
+            return self.valobj.num_children
         return self.valobj.GetIndexOfChildWithName(name)
 
     def get_child_at_index(self, index):