Further integration of Dispatch into Foundation

[seabaylea]

This makes some minor changes that gets us closer to having Dispatch built and included in CoreFoundation and Foundation, including:

• Switching CFRunLoop to use the new 4CF functions in libdispatch
  Note that these APIs are only available in the `experimental/foundation' branch of libdispatch.
• Adding a basic implementation for pthread_main_np()
  There isn't an implementation in Glibc and there isn't a shim today. We should potentially move the implementation to the Glibc overlay.
• Updated the build scripts to include Dispatch in Foundation as well as CoreFoundation
  This is still commented out as there's a number of compile errors in NSOperation and the incoming NSURLSession code - we should fix those up before uncommenting this code.

The changes should allow anyone to build a toolchain with Dispatch enabled by:

1. Extracting the experimental/foundation branch of swift-corelibs-libdispatch
2. Uncommenting the code in build.py
3. Running a build with the libdispatch and install-libdispatch flags added

[modocache]

I don't have a ton of context on the changes in this pull request, but thought I'd chime in with a tiny nit-pick: Is there a reason you uncommented this line? That's fine of course, but Python requires if to contain at least one expression. To do nothing, you may use pass:

if "LIBDISPATCH_SOURCE_DIR" in Configuration.current.variables:
    pass

I think if you run this as-is, it would raise a syntax error of some kind.

[seabaylea]

Thanks for picking that up. Looks like I missed a line when I re-commented out the block as building Dispatch into Foundation currently hits compile errors in NSOperation (due to some Dispatch overlay work that needs fixing up).

Now fixed