We still have a bunch of redeclarations of Dispatch functions to avoid
the automatic bridging of dispatch_data_t and dispatch_block_t, but
mostly this is a vast reduction in complexity (and increase in safety).
Loading a Clang module eagerly brings in overlays for anything it re-exports,
but this is a problem for these new "shim header" modules, which generally
import the underlying module for an overlay and are in turn imported by the
overlay. That means that when we try to import an overlay, we'll end up with
a circular reference before it's done loading all its dependencies. Break the
cycle by not exporting anything from these modules, which are mostly just an
implementation detail anyway.
* Fix DispatchSourceSignal initialisation such that it no longer
registers for the wrong source type.
* Remove (group:) option from DispatchWorkItem, introduce group
options to `.async` methods that accept DispatchWorkItem.
* Rename `DispatchSourceType` to `DispatchSourceProtocol`
* Rework DispatchQueue attributes and flags into a less confusing
approach.
* Fixes:
SR-1817, SR-1771, SR-1770, SR-1769
<rdar://problem/26725156> <rdar://problem/26873917>
<rdar://problem/26918843> <rdar://problem/26810149>
<rdar://problem/27117023> <rdar://problem/27121422>
<rdar://problem/27236887> <rdar://problem/27337555>
