[Distributed] Fix warning about comparisons like 'X<=Y<=Z'
Avoids the following warning:
```
/Users/ktoso/code/swift-project/swift/lib/Sema/CodeSynthesisDistributedActor.cpp:860:43: warning: comparisons like 'X<=Y<=Z' don't have their mathematical meaning [-Wparentheses]
860 | assert(idIdx < actorSystemIdx < unownedExecutorIdx &&
```
