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

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

Locate the Swift declarations for DispatchWorkItemsFlags and DispatchQueue, then reproduce the Swift 6.2 Linux build error shown in the issue. Check the existing Dispatch API declarations and related Sendable changes referenced by issues #779, #787, #845, and #847; done means the affected GRDB build no longer reports these non-Sendable types.