Scoped GrowsBackwards-optimized erase to only be applied
when erase starts at the beginning of the element range.
In other cases, old "left-shifting" erase is used to align
with std::vector::erase invalidation policy
Task-number: QTBUG-84320
Change-Id: I2e7f3b96b056bc371119eb2d36cc7c74af52c394
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>