[CMake] Providing a CMake cache for 3-stage builds

This cache file can be used to generate a 3-stage clang build. You can configure using the following CMake command:

cmake -C <path to clang>/cmake/caches/3-stage.cmake -G Ninja <path to llvm>

You can then run "ninja stage3-clang" to build stage1, stage2 and stage3 clangs.

This is useful for finding non-determinism the compiler by verifying that stage2 and stage3 are identical.

llvm-svn: 260201