[WIP][Concurrency] Optimize Void task group, to not store completed tasks

[ktoso]

This optimization work is to enable "infinite" task groups which just keep adding tasks "forever" but don't really need child task return values - for example, when implementing an RPC or web server, one might want to use a task group in order to stay inside structured concurrency for benefits of efficiency, task-local inheritance and canceling all in-flight rpcs when the server is cancelled.

Naively trying to handle such server case with:

    try await withThrowingTaskGroup(of: Void.self) { group in
        for try await query in dnsQuerySequence {
            group.addTask {
                handleQuery(query)
            }
        }
    }

--

Revision 2:

    try await withThrowingTaskGroup(of: Void.self, discardResults: true) { group in
        for try await query in dnsQuerySequence {
            group.addTask {
                handleQuery(query)
            }
            group.next() // always `nil`
        }
    }

Resolves rdar://101965913

[ktoso]

Superseeded by #62361