Pending SE evolution: swiftlang/swift-evolution#2990 This makes Task.init and friends adopt typed throws, and also drops discardable result from the throwing APIs

After a false start, @ktoso and I are back at it. We'd would like to make another attempt at adopting typed throws in the Task creation APIs and make it more difficult to ignore thrown errors accidentally. The spirit of the changes are identical to the previous proposal, but have been expanded to accommodate all the new Task APIs. Just to recap...