Add missing Sendable conformances

[kcede]

Description

In the SDK that ships with Xcode, these types are marked as Sendable, which causes source compatibility issues.

Adds missing Sendable conformance to the following types:

• DispatchQueue.Attributes
• DispatchQueue.GlobalQueuePriority
• DispatchQueue.AutoreleaseFrequency
• DispatchWorkItemFlags
• DispatchIO.StreamType
• DispatchIO.CloseFlags
• DispatchIO.IntervalFlags
• DispatchData
• DispatchData.Deallocator
• DispatchDataIterator

Related issues: #779, #910

Reproduction

import Dispatch

@main
struct swift_dispatch_sendable {
    static func main() async {
        let sendables: [Sendable] = [
            DispatchQueue.Attributes.concurrent,
            DispatchQueue.GlobalQueuePriority.background,
            DispatchQueue.AutoreleaseFrequency.inherit,
            DispatchWorkItemFlags.assignCurrentContext,
            DispatchIO.StreamType.random,
            DispatchIO.CloseFlags.stop,
            DispatchIO.IntervalFlags.strictInterval,
            DispatchData.empty,
            DispatchData.Deallocator.free,
            DispatchData.empty.makeIterator(), // DispatchDataIterator
        ]
    }
}

Expected behavior

The example program should compile without errors.

[trbernstein]

This PR is intended to align non-Darwin Dispatch sendability with Darwin behavior.
It appears to address the same class of mismatch discussed in #779, #845, and #847.
The practical effect is that code passing strict concurrency checks on macOS can still fail on Linux due to missing Sendable conformances.

Could a maintainer confirm whether the direction here is acceptable, or whether this should be split into smaller PRs / gated differently?

Edit: While reviewing this again I noticed that DispatchDataIterator currently has an @unchecked Sendable conformance.
That might warrant a closer look, since it could indicate internal mutability or assumptions about thread-safety.

[ktoso]

@swift-ci please smoke test

[ktoso]

I think this is reasonable, thank you

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please smoke test