Otherwise, we would generate inconsistent vtable layouts for classes
with static properties that have attached wrappers. The reason is that
we normally force synthesis of the backing storage and storage wrapper
for each instance property wrapper as part of computing the lowered
stored properties.
However, there was no such forcing for static properties. But since a
static stored property (with an attached wrapper or otherwise) must be
'final', the real fix is to just ensure that the 'final' bit propagates
to the storage wrapper as well.
The backing storage property was already always final, so the issue
did not arise there.
Fixes <rdar://problem/59522703>, <https://bugs.swift.org/browse/SR-12429>.
