This allows catching OOB accesses inside `unique_ptr<T[]>` when the size
of the allocation is known. The size of the allocation can be known when
the unique_ptr has been created with make_unique & friends or when the
type necessitates an array cookie before the allocation.
This is a re-aplpication of 45a09d181 which had been reverted in
f11abac6 due to unrelated CI failures.