gdbsupport: make reference_to_pointer_iterator's constructor non-variadic

For the same reason as the previous patches (making things easier to
understand, at the cost of being more explicit), change
reference_to_pointer_iterator's constructor to take an already built
underlying iterator.

There are no uses of this utility in the code base right now, so no
callers to update.

Change-Id: Ifadb4c3a5ee53a8d0eb6c0d79efcc92271492e8e
Approved-By: Tom Tromey <tom@tromey.com>

gdbsupport/reference-to-pointer-iterator.h

@@ -38,9 +38,8 @@ struct reference_to_pointer_iterator
 
   /* Construct a reference_to_pointer_iterator, passing args to the underlying
      iterator.  */
-  template <typename... Args>
-  reference_to_pointer_iterator (Args &&...args)
-    : m_it (std::forward<Args> (args)...)
+  explicit reference_to_pointer_iterator (IteratorType it)
+    : m_it (std::move (it))
   {}
 
   /* Create a past-the-end iterator.