QVarLengthArray is missing std::launder()

See epic for more information.

QVLA is using a char array as internal storage, so we need to launder() at least when elements could be residing in internal storage ("std::optional" pattern in the epic), but not when elements are from heap storage!

Update 2023-02-15: C++17 needs the launder() also when placing new objects, just in case the value_type contains const or reference members, and we emplace_back() over a previously pop_back()'ed element.