Lower the async task slab allocation to 1000 bytes.
Due to malloc quanta rounding, 1024-byte async task allocation slabs
actually end up allocating 1536 bytes on Darwin. Instead, use 1000-byte
slabs.
Fixes rdar://81181856.
(cherry picked from commit 64c670745a5bbf4758aa98e96996a5cf53dac344)
