This is in order to avoid errors in complete concurrency checking mnode
in distributed funcs, or rather their thunks, as there is isolation
boundary crossing happening when we pass a value to a distributed func.
This is because as we do this, we pass it to a nonisolated thunk:
```
nonisolated func THUNK(param: Thing) {
if remote {
...
} else {
await self.realFunc(param)
}
}
```
So what happens here is that the Thing would become isolated to the
task and we get a bad isolation crossing as we pass it along to the
"real func".
Sending values into the distributed thunk is the right thing to do to
resolve this problem: `nonisolated func THUNK(param: sending Thing) {}`
Resolves rdar://126577527
e650dc00df