[Concurrency][SE-0381] DiscardingTaskGroup

[ktoso]

Defined by SE-0381: https://github.com/apple/swift-evolution/blob/main/proposals/0381-task-group-discard-results.md

---

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 withThrowingDiscardingTaskGroup() { group in
        for try await query in dnsQuerySequence {
            group.addTask {
                handleQuery(query)
            }
        }
    }

Resolves rdar://101965913

[ktoso]

@swift-ci please smoke test

[ktoso]

Uff, good, caught an accidental dumb ABI problem, fixed and this should be a good first cut.

I'll finish up error and cancellation handling a bit more tomorrow.

@swift-ci please smoke test

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please smoke test

[ktoso]

This is "complete" but there's a PR with more code-reuse incoming...

[ktoso]

Replaced by #62914