We no longer attempt to convert timestamps from the passed-in `Clock`
in order to allow any clock to work with any executor. Instead,
executors that do not recognise a clock should call the `enqueue`
function on that `Clock`, which lets the `Clock` itself decide how
to proceed.
Additionally, rename `SchedulableExecutor` to `SchedulingExecutor`.
In task-to-thread concurrency mode, `_getMainExecutorAsSerialExecutor`
returns `SerialExecutorRef::generic()`, which is, give or take, NULL.
Unfortunately we'd declared it as returning `any SerialExecutor`,
rather than `(any SerialExecutor)?`, and then the `getMainExecutor()`
function was calling `asUnownedSerialExecutor()` on it, which then
crashes.
rdar://153152063
Custom main and global executors work hasn't passed Swift Evolution yet,
so we need to avoid leaking it as API until it does.
To that end, underscore all the things.
rdar://151147606
Remove `supportsScheduling` in favour of a type-based approach.
Update the storage for `ClockTraits` to `UInt32`.
Adjust ordering of executors for `currentExecutor`.
rdar://141348916
Added an `-executor-factory` argument to the compiler to let you safely
specify the executors you wish to use (by naming a type that returns
them).
Also added some tests of the new functionality.
rdar://141348916
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
