Lower the async task slab allocation to 1000 bytes.
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.

roambotics/swift,ahoppen/swift,roambotics/swift,xwu/swift,atrick/swift,benlangmuir/swift,hooman/swift,glessard/swift,glessard/swift,rudkx/swift,apple/swift,gregomni/swift,atrick/swift,gregomni/swift,benlangmuir/swift,apple/swift,hooman/swift,apple/swift,rudkx/swift,hooman/swift,glessard/swift,roambotics/swift,atrick/swift,gregomni/swift,ahoppen/swift,benlangmuir/swift,roambotics/swift,ahoppen/swift,xwu/swift,ahoppen/swift,atrick/swift,ahoppen/swift,ahoppen/swift,gregomni/swift,roambotics/swift,apple/swift,xwu/swift,xwu/swift,JGiola/swift,rudkx/swift,JGiola/swift,glessard/swift,JGiola/swift,rudkx/swift,rudkx/swift,gregomni/swift,xwu/swift,benlangmuir/swift,hooman/swift,glessard/swift,hooman/swift,hooman/swift,JGiola/swift,xwu/swift,hooman/swift,apple/swift,apple/swift,gregomni/swift,atrick/swift,glessard/swift,rudkx/swift,roambotics/swift,JGiola/swift,benlangmuir/swift,benlangmuir/swift,atrick/swift,JGiola/swift,xwu/swift