[Concurrency] Never apply default main actor isolation to local declarations.

[hborla]

Otherwise, code like this mysteriously fails in -default-isolation MainActor (in this case, because the group parameter is accidentally inferred as @MainActor):

func example() async {
  await withTaskGroup { group in
    group.addTask {}
    await group.next() // error
  }
}

Resolves: rdar://153561279

[hborla]

@swift-ci please smoke test

[ktoso]

Ouch, good catch lgtm

[hborla]

Closing in favor of #83175