QFlags doesn't support large (> sizeof(int)) enums

First caught in https://codereview.qt-project.org/c/qt/qtbase/+/35764, with plans to introduce support in 5.1. Then we removed indications of that plan in https://codereview.qt-project.org/c/qt/qtbase/+/54720 (not sure why, though).

Unless we need a guarantee that QFlags<T> is the same size for all T, using underlying_type<T> instead of int for sizeof(T) > sizeof(int) seems like an easy way out.