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

this issue originally filed at https://github.com/swiftlang/swift/issues/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

Start with the DispatchQoS and DispatchSpecificKey declarations exposed by the Dispatch module and compare their Sendable conformances on macOS and Linux. Reproduce the issue with the provided swiftc example targeting x86_64-unknown-linux-gnu; done means both types have matching cross-platform conformances and the example compiles consistently.