Make _predatesConcurrency ABI/API stable to add
Make _predatesConcurrency ABI/API stable to add

Using _predatesConcurrency is what prevents adding `@Sendable` to things
from breaking the ABI. Yes, technically adding it will break anything
that had `@Sendable` before and is now pre-dating concurrency. After
discussing with Doug, this was the direction we chose to take.

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