[Concurrency] Fix swift_taskGroup_initializeWithFlags availability

[ktoso]

Description: The new runtime function swift_taskGroup_initializeWithFlags introduced to support discarding task group initialization has wrong availability. It must have availability equal to when it was introduced, but had the "general" concurrency one. Its availability must be 5.9. If we don't do this, then code like this:

if #available(iOS 17.0, *) {
    await withDiscardingTaskGroup {
        $0.addTask {
            try? await Task.sleep(for: .seconds(1))
        }
    }
}

can crash when compiled against iOS 16 target, since it'd attempt to strong refer to the symbol, but it's missing, yielding a Symbol not found: _swift_taskGroup_initializeWithFlags

Risk: Low, the method we're changing availability of cannot be called from user code, and is only called in IRGen generated code inside a method that is properly availability annotated already.
Reward: High, without the fix adopting discarding task groups is limited in apps which also need to support older versions of iOS which do not have this API, as the "usual" if #available guarding does not prevent the crash.
Review by: @rjmccall @mikeash
Testing: CI testing
Radar: rdar://112469076

[ktoso]

@swift-ci please smoke test

[rjmccall]

LGTM