[Concurrency] Relax task-local mis-use prevention in task groups

[ktoso]

We have a mechanism protecting task group child tasks from accessing values in "outer scope" if they are defined like this:

await withTaskGroup { group in 
  await $local.withValue(value) {  // push "value"
    group.addTask {} // would refer to "value" by "direct pointer" to parent task
  } // pop value ---- if we allowed the above "direct pointer" the child task now refers to trash memory
}  

So to prevent this, today, we crash eagerly when we detect this.

The problem:

We crash to aggresively, also falsely triggering on this:

await withTaskGroup { group in 
  await $local.withValue(value) {  // does crash, but shouldn't
  }
  group.addTask {} 
}  

This patch solves this behavior by more precise runtime detecting of the specific problem situation -- we now crash inside the addTask and NOT inside the withValue.

Future direction: We could start copying defensively when we notice the "bad situation" rather than just crash. @FranzBusch was arguing that behavior would be preferable -- because we cannot always control the shapes of APIs, and if someone were to offer a synchronous callback API and they had set a task local value inside a task group "by accident" without even knowing about it, today we would crash - causing weird non-local interactions.

I'll see if I can pull off the copying "only when we have to in the bad-bad and rare case".

resolves rdar://127156303

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please smoke test

[ktoso]

replaced by #73978