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.