This allows programs to target older OSes while using Concurrency behind an availability check. When targeting older OSes, the symbols are weak-linked and the compiler will require the use of Concurrency features to be guarded by an availability check.
rdar://75850003
- introduce UnsafeCurrentTask
- implement Hashable, Equatable on tasks
- assume we'll have a way to get a task from sync context
- Task.Handle now has a Failure type as well
- Task.Handle.getResult