Darwin OSes support vouchers, which are key/value sets that can be adopted on a thread to influence its execution, or sent to another process. APIs like Dispatch propagate vouchers to worker threads when running async code. This change makes Swift Concurrency do the same.
The change consists of a few different parts:
1. A set of shims (in VoucherShims.h) which provides declarations for the necessary calls when they're not available from the SDK, and stub implementations for non-Darwin platforms.
2. One of Job's reserved fields is now used to store the voucher associated with a job.
3. Jobs grab the current thread's voucher when they're created.
4. A VoucherManager class manages adoption of vouchers when running a Job, and replacing vouchers in suspended tasks.
5. A VoucherManager instance is maintained in ExecutionTrackingInfo, and is updated as necessary throughout a Job/Task's lifecycle.
rdar://76080222
