QArrayDataPointer::assign(it, it): apply lessons learned in imlementing QString::assign(it, it) for UTF-8

This is a follow-up task on QTBUG-114208.

It looks like QString::assign_helper_char8() will end up with a superior structure than what we have currently have in QArrayDataPointer::assign(it, it) neé QList::assign(it, it). In particular, where the latter is falling back to a loop over emplace_back() when the capacity() is exhausted, the former will realloc(Grow), reset the output buffer pointers, and continue.

Acceptance criteria:

• Check under which conditions we can apply this optimization
• and apply it in the code.