We were missing a field in `SwiftJob`, which broke various things. To
avoid that problem in future, this PR adds a set of static asserts to
check the layout of various structures and that we're using the same
values as well.
Also added some functions to the ABI, and fixed things so that if you
enable the debug logging the library still builds (there was an extra
`_` in `Actor.cpp` that caused a build failure).
Finally, renamed `Hooks.cpp` to `ConcurrencyHooks.cpp`.
rdar://135380149
