This was previously not required, but the patch to introduce a new
size-based vector layout unintentionally added this new requirement. We
almost certainly not want to promise this guarantee going forward, but
we should actually land this change explicitly and consider the
transition story, not do it as a fallout of another refactoring.
Fixes#210732
(cherry picked from commit 17ac8fdd95283110a14f1de8c15a0fc661119296)