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

[tayloraswift]

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

[jamieQ]

this seems like a duplicate of: swiftlang/swift-corelibs-libdispatch#845 (or is at least closely related)

[AnthonyLatsis]

@parkera Are you able to transfer this?

[ktoso]

@rokhinip would have the power to do so; I don't sadly.

[ktoso]

It seems we can't transfer issues cross org; @tayloraswift please open this on corelibs dispatch, we can't do anything about it in swiftlang/swift.

[AnthonyLatsis]

It seems we can't transfer issues cross org

Ah, yeah, I thought GCD made it into swiftlang too.