I had originally thought that we'd get away without this because the
executor construction generally happens in the async `main` function,
but of course if a program doesn't already use Swift Concurrency,
and someone uses it from a plug-in or library it loads, there's a
risk that we might have a race condition.
rdar://149346132
Reorganise the Concurrency code so that it's possible to completely
implement executors (both main and global) in Swift.
Provide API to choose the desired executors for your application.
Also make `Task.Sleep` wait using the current executor, not the global
executor, and expose APIs on `Clock` to allow for conversion between
time bases.
rdar://141348916
