[CMake] Add cache scripts for Apple-style clang builds.

Summary:
These CMake cache scripts are my first pass at replicating Apple's packaging logic from autoconf. They can be used on any Darwin machine to approximate an Apple Clang build.

The included README file includes documentation and a sample CMake invocation.

Reviewers: chandlerc, echristo

Subscribers: echristo, cfe-commits

Differential Revision: http://reviews.llvm.org/D12817

llvm-svn: 247726