[cmake] Add an option to prefer public SDK in find_darwin_sdk_dir

Adds a CMake option DARWIN_PREFER_PUBLIC_SDK, off by default. When on, this prefers to use the public SDK, even when an internal one is present. With this, it's easy to emulate a build that the public buildbots are doing.

Differential Revision: https://reviews.llvm.org/D35071

llvm-svn: 307330