Fix compilation issues with HAS_DISPATCH on Linux

[seabaylea]

As part of the prep work I'm doing with @dgrove-oss to enable Dispatch, I've done some testing of compiling swift-corelibs-foundation with __HAS_DISPATCH__ enabled. That's highlighted a number of compile errors which this PR resolves.

Enabling __HAS_DISPATCH__ will still result in linker errors, which we'll look to resolve once we have the Dispatch overlay built into lib dispatch on Linux (libdispatch PR #43, and made the necessary modifications to the foundation build scripts.

[phausler]

why don't we have /usr/local/include/dispatch/private.h? can we just pass across private header paths if it isn't installed into a staging sysroot?

and I am not certain that /usr/include/dispatch/dispatch.h missing is a good thing. If that is not actually missing then perhaps we should just move the private.h out only.

[seabaylea]

We don't have a dispatch/private.h on Linux so it needs to be removed (at least for now). There's also no direct need to reference dispatch/dispatch.h from CFInternal.h, but I'll, update to retain the reference.

[dgrove-oss]

There's a workaround for dispatch/private.h in the pending overlay pull requests. Basically it exploits the symlink that is made by configure from $(top_bindir)/tests/dispatch to $(top_srcdir)/private to allow an include of dispatch/private.h to succeed. Personally, I think this is a bit quirky, but it works (and is consistent with how the include of dispatch/private.h is made to work when compiling libdispatch's test cases on linux).

[seabaylea]

Sorry, I realised I'd created the pull request from and old working branch in my repo. I've updated the PR with the correct changes.

The reference to dispatch/private.h in CFInternal.h has been removed as there isn't an equivalent on Linux. There may be a few issues we'll need to work through on linking though once @dgrove-oss gets his build changes in.

[seabaylea]

Thanks @dgrove-oss . Given that the reference to private.h is still protected under __HAS_DISPATCH__ (which isn't enabled yet) then I'll revert to including dispatch/private.h as it won't cause a breakage out of the box, and will be ready for when your pending overlay pull requests come in.

[seabaylea]

@phausler Changes made and commits squashed in place. Could you take another look?

[phausler]

looks good to me

[seabaylea]

Thanks @phausler !