This replaces 8711adb7

if a temptable field is created for some json expression (is_json_type()
returns true), make this temptable field a proper json field.

A field is a json field (see Item_field::is_json_type()) if it
has a CHECK constraint of JSON_VALID(field).

Note that it will never be actually checked for temptable fields,
so it won't cause a run-time slowdown.