Rename private member of FrameDecorator

In Python, a member name starting with "__" is specially handled to
make it "more private" to the class -- it isn't truly private, but it
is renamed to make it less likely to be reused by mistake.  This patch
ensures that this is done for the private method of FrameDecorator.

gdb/python/lib/gdb/FrameDecorator.py

@@ -53,7 +53,7 @@ def __init__(self, base):
         self._base = base
 
     @staticmethod
-    def _is_limited_frame(frame):
+    def __is_limited_frame(frame):
         """Internal utility to determine if the frame is special or
         limited."""
         sal = frame.find_sal()
@@ -148,7 +148,7 @@ def frame_args(self):
             return self._base.frame_args()
 
         frame = self.inferior_frame()
-        if self._is_limited_frame(frame):
+        if self.__is_limited_frame(frame):
             return None
 
         args = FrameVars(frame)
@@ -164,7 +164,7 @@ def frame_locals(self):
             return self._base.frame_locals()
 
         frame = self.inferior_frame()
-        if self._is_limited_frame(frame):
+        if self.__is_limited_frame(frame):
             return None
 
         args = FrameVars(frame)
@@ -179,7 +179,7 @@ def line(self):
             return self._base.line()
 
         frame = self.inferior_frame()
-        if self._is_limited_frame(frame):
+        if self.__is_limited_frame(frame):
             return None
 
         sal = frame.find_sal()