`DispatchQoS` and `DispatchSpecificKey` should have consistent `Sendable` conformances between macOS and linux

[tayloraswift]

this issue originally filed at swiftlang/swift#76773

Description

the DispatchQoS and DispatchSpecificKey are conditionally Sendable on macOS, but not on linux for some reason, which creates a needless portability barrier for developers that aim to support both platforms.

Reproduction

import Dispatch
func f()
{
    let x:DispatchQoS = { fatalError() }()
    Task.init
    {
        let y:DispatchQoS = x
        let _ = y
    }
    Task.init
    {
        let y:DispatchQoS = x
        let _ = y
    }
}

Expected behavior

the types should have consistent conformances across platforms

Environment

$ swiftc --version
Swift version 6.0.1 (swift-6.0.1-RELEASE)
Target: x86_64-unknown-linux-gnu

Additional information

the original context was attempting to build GRDB.swift on linux