Updates to CF imports to swift-corelibs-foundation

I just recently put up a PR that brings in the Apple changes from CoreFoundation to swift-corelibs-foundation. This brings our CF backing for SCLF to be in line with the Sierra/iOS10 release. I was unable to test other platforms like Android, Cygwin, or other architectures like Raspberry Pi or SystemZ so some of the changes that I made may have regressed some of our ports.

One major change that I had to make due to other constraints was a re-work of some of the string constants. This makes the Android symbolic definition for NSConstantString no longer work and we should consider a more uniform method to do that similar to the Ubuntu targets (which was modeled after the Darwin version). If you have time; please review the cross platform changes that were made on https://github.com/apple/swift-corelibs-foundation/pull/709\. I will be making some additional changes to gate ICU versions and some other key portions for making certain this builds on all platforms.

Thanks for your time,