Previously we've checked whether an ID is available by computing the
full list of all known available IDs (converting each from NSString to
QBA in the process) in order to compare the offered ID to each of
those in turn. This is a significant performance bottle-neck
Simply trying to construct the NSTimeZone instance should suffice to
determine whether there is such a zone; if the ID is not available, we
apparently get back a null pointer, which is easy enough to test for.
Pick-to: 6.6 6.5
Fixes: QTBUG-118759
Change-Id: Ib272c96e25db9640490299c638e2e9196a58712f
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
be55fbede9