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>