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>

single_include/nlohmann/json.hpp

@@ -21020,10 +21020,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 #endif
     }
 
-    iterator set_parents(iterator it, typename iterator::difference_type count_set_parents)
+    iterator set_parents(iterator it, std::ptrdiff_t count_set_parents)
     {
 #if JSON_DIAGNOSTICS
-        for (typename iterator::difference_type i = 0; i < count_set_parents; ++i)
+        for (std::ptrdiff_t i = 0; i < count_set_parents; ++i)
         {
             (it + i)->m_parent = this;
         }