This patch builds clang-extra-tools on macOS for the
clang-tidy binary. The script "run-clang-tidy.py" is
also harvested because using the CMAKE_C[XX]_CLANG_TIDY
option can miss out some files (like makesrna), and using the
script is faster as it does not compile the files.
Thanks to @Ray Molenkamp (LazyDodo) for the base patch.
It is based on patch D8502. I had come up with https://pasteall.org/VwhU/slim which could be
cleaner if we used the unified llvm source dir, and then use LLVM_ENABLE_PROJECTS.
The version of clang tidy that macOS developers get is 9. It is tied to LLVM library update
and thus will need us to change .clang-format to avoid newer changes.
For newer versions of clang tidy, build from source or use a package manager or
official website of llvm to get the binaries.