gdbsupport: add iterator_range::empty

Add iterator_range::empty, indicating if the range is empty.  This is
used in the following patch.

Change-Id: I1e6c873e635c2bb0ce5aaea2a176470970f6d7ac
Approved-By: Tom Tromey <tom@tromey.com>

gdbsupport/iterator-range.h

@@ -56,6 +56,10 @@ struct iterator_range
   std::size_t size () const
   { return std::distance (m_begin, m_end); }
 
+  /* Return true if this range is empty.  */
+  bool empty () const
+  { return m_begin == m_end; }
+
 private:
   IteratorType m_begin, m_end;
 };