It's a proof of concept for T90379.. a script can be added to download the llvm binaries
and set up toolchain automatically. I didn't do it out of fear of wasted
effort.

The bl.cmake shouldn't be in git, it's for demonstration. Devs would
have to edit it locally to match their system.

While building, I didn't have LLVM clang in path or any ENV vars
referring to it. No system disruption should happen.

If devs want, ofcourse they can change system wide compiler by
Xcode app menu > Toolchains > choosing LLVM one.

Needs to be tested

• on buildbot if codesign works or not,
• if blender launches on other systems or not etc.
• if ccache option works or not.
• if works with xcode 10 or not