Rework of Dispatch overlay for Linux

The lack of Objective-C and associated bridging
when importing the libdispatch header files into Swift
caused a number of problems including
  (a) missing ref count operations in Swift compiled code
  (b) SR-739, dispatch types not being upcastable to dispatch_object_t
  (c) SR-730, dispatch types not being upcastable to AnyObject
  (d) SR-737, dispatch types being imported as COpaquePointer

This commit fixes all of these issues by injecting a complete
Swift overlay layer that wraps the native libdispatch objects
and native APIs.

The C header files are now imported into a distinct CDispatch module
that is not made available to Swift client code (which continues to
import Dispatch). Code is added to Dispatch.swift to mirror the
native CDispatch APIs up to the Swift Dispatch API and to wrap/unwrap
values as needed across that boundary.

This wrapping layer does add some minor space and time overheads,
but after extensively exploring implementing the Swift object model
within the C code of libdispatch (to avoid introducing this overhead),
I concluded that the necessary changes to libdispatch would be fairly
invasive, and therefore were not justified at this point in the
development of the Swift/Linux port of libdispatch.

Author: dgrove-oss

dispatch/module.map

@@ -1,4 +1,11 @@
-module Dispatch [system] {
+module Dispatch {
+    requires blocks
+    export *
+    link "dispatch"
+    link "BlocksRuntime"
+}
+
+module CDispatch [system] {
     umbrella header "dispatch.h"
     requires blocks
     export *