Swap diagnostic positions in basic_json::swap()

basic_json::swap() (and the friend swap() that forwards to it) only
exchanged m_data.m_type/m_data.m_value, leaving start_position/end_position
untouched under JSON_DIAGNOSTIC_POSITIONS. This is inconsistent with
copy-assignment's operator=(basic_json), which swaps positions as part of
its copy-and-swap implementation, so after swap(a, b) each value ended up
with the other value's content but its own original position.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json.hpp

@@ -3547,6 +3547,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::swap(m_data.m_type, other.m_data.m_type);
         std::swap(m_data.m_value, other.m_data.m_value);
 
+#if JSON_DIAGNOSTIC_POSITIONS
+        std::swap(start_position, other.start_position);
+        std::swap(end_position, other.end_position);
+#endif
+
         set_parents();
         other.set_parents();
         assert_invariant();