Fix incomplete-type error in set_parents with ordered_json (#5167)

When iteration_proxy_value<iter_impl<ordered_json>> appears in a context
that requires it to be complete (function or lambda parameter), the
compiler instantiates basic_json<ordered_map> and walks into

    set_parents(iterator, typename iterator::difference_type)

while iterator is still incomplete, failing with "invalid use of
incomplete type".

basic_json::difference_type is already std::ptrdiff_t, so just naming
the underlying type directly avoids the dependent lookup. Behavior and
ABI are unchanged. This was the approach suggested in the issue thread.

Added a regression case in unit-ordered_json.cpp using the same trigger
pattern (lambda parameter naming the proxy type).

Fixes #3732

Signed-off-by: Akhilesh Arora <akhildawra@gmail.com>