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
We decided that using a magic typealias to set the executor factory was better
than using a compiler option. Remove the `-executor-factory` option, and replace
by looking up the `DefaultExecutorFactory` type, first in the main module, and
then if that fails in Concurrency.
rdar://149058236
`CooperativeExecutor` should fall back to heap allocations if the task
allocator isn't available when it's enqueuing delayed jobs.
Also, remove the reference to `DispatchGlobalExecutor` from the
custom executor test.
rdar://141348916
EventableExecutor is being removed, for now, but hopefully will
return in some form in the future.
The `asSchedulable` implementation needs to change for reasons of
ABI stability.
rdar://141348916
Rename `DispatchTaskExecutor` to `DispatchGlobalTaskExecutor` as we
may want to use the former for an executor that runs things on an
arbitrary Dispatch queue.
Rename `DispatchExecutor` to `DispatchExecutorProtocol`; again, we
might want the name for something else.
Add `@Sendable` attribute to `registerEvent`.
Fix missing `extern "C" SWIFT_CC(swift)` on `_swift_exit` (merge
error).
Remove stray whitespace from `CMakeLists.txt`
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
