Add an MACOS_VERSION_UNKNOWN_NEWER enum value for OS X versions above 10.10.

We recently had a broken version check because an newer OS X version is treated as MACOS_VERSION_UNKNOWN which is less than all the defined values. Let's have a separate enum value for unknown but newer versions, so the ">=" and "<=" version checks still work even in upcoming OS X releases.

Reviewed at http://reviews.llvm.org/D6137