Add `Sendable` conformance to `DispatchWorkItemsFlags` and `DispatchQueue`

[alephao]

Similar to #779, #787, #845 and #847.

• DispatchWorkItemsFlags has no mutable state
• I guess DispatchQueue is Sendable on iOS, so maybe it could be Sendable on Linux too?

This error is triggered when building GRDB

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302.2 25 |     private let flags: DispatchWorkItemFlags
302.2    |                 `- error: stored property 'flags' of 'Sendable'-conforming class 'DispatchQueueExecutor' has non-sendable type 'DispatchWorkItemFlags'
302.2 26 |     
302.2 27 |     init(queue: DispatchQueue, flags: DispatchWorkItemFlags) {
302.2 
302.2 Dispatch.DispatchWorkItemFlags:1:15: note: struct 'DispatchWorkItemFlags' does not conform to the 'Sendable' protocol
302.2  1 | public struct DispatchWorkItemFlags : OptionSet, RawRepresentable {
302.2    |               `- note: struct 'DispatchWorkItemFlags' does not conform to the 'Sendable' protocol
302.2  2 |     public let rawValue: UInt
302.2  3 |     public init(rawValue: UInt)
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Environment

Swift version 6.2 (swift-6.2-RELEASE)
Target: aarch64-unknown-linux-gnu