QDateTime's comparisons just compare milliseconds when both values are
local times and their statuses (hence DST-ness) match. It can do the
same for time-zones. While doing the same for UTC and fixed offsets
wins nothing, it also costs nothing, given that we're already checking
the spec.
Task-number: QTBUG-75585
Change-Id: Ib6d824569aba8def2f1319ef3a11cca6869a5b5e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>