Preprogam QHashPrivate::Span::Entry as a union for Qt 7, bootstrap, to avoid laundering on every node access

See comments on QTBUG-111043 for an analysis.

TL;DR:

• Entry uses an ex-std::aligned_storage_t to store either a Node or a uchar
• so it should be a union, and then std::launder()'ing wouldn't be necessary on each node access
• making it a union will = delete its dtor if Node (~ pair<Key,Value>) is not std::is_trivially_destructible
• Entry doesn't know whether it contains a Node or not (that info is in Span::offsets), so we can't add a user-defined ~Entry()
• the existing code uses new[] and delete[] to (de)allocate Span::entries, so we need Entry to be std::destructible
• we cannot use malloc() and free() instead because old code will continue to use new[] and delete[], and mixing the two is UB

In order to avoid forgetting this come Qt 7, we should pre-program for QT_BOOTSTRAP || QT_VERSION_CHECK(7, 0, 0).