[stdlib] replace uses of assert in the Concurrency module
[stdlib] replace uses of assert in the Concurrency module

- we define a local equialent to `_internalInvariant` that will compile to nothing in release builds
- `assert` was still detectable in release builds, presumably costing execution time

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