🍒[5.9] Fix pointer auth issues for discarding task group operation pointer

[ktoso]

Description: Pointer authentication used in swift_create_task works with closures that return a generic type and using a closure like () -> () in the way discarding task group does was causing pointer auth failures on arm64e preventing adoption of DiscardingTaskGroup on that platform. The solution is to have fix the closure signature, but just discard the returned value.
Risk: Medium, this changes the signature of the addTask APIs from () -> () to () -> T, however is source compatible. Adopters depending on ABI using a discarding group would have to be rebuilt. Code change wise, this is low risk.
Review by: @mikeash
Testing: PR testing and have manually verified the pointer auth failure goes away on arm64e
Original PR: #65220
Radar: rdar://107574868

[ktoso]

@swift-ci please test

[ktoso]

This actually was green already but I wrongly closed the PR...

We have now confirmed this DOES fix the issue.

[ktoso]

@swift-ci please test

[FranzBusch]

Very interesting bug @ktoso! I sadly have no idea how to fix the issue of getting signed pointers for the () -> () case but I just wanted to chime in since I don't know if we should go forward with the current proposed fix.

This changes the API of the addTask method where one can now write the following code and not get any warnings

await withDiscardingTaskGroup() { group in
    group.addTask {
        return 1 // This currently produces a warning since the return type is Void
    }
}

Would love if we could find another way to fix the arm64e issue without changing the API

[ktoso]

Yeah that's unfortunate, thanks for mentioning this @FranzBusch -- I'll still see if we can do something about it perhaps; It's not a big deal to be honest though I think.