How to build the latest clang-tidy?

I've tried to build clang-tidy from sources but it complains about an undefined CMake command:

CMake Error at clang-apply-replacements/CMakeLists.txt:5 (add_clang_library):
  Unknown CMake command "add_clang_library".


CMake Warning (dev) in CMakeLists.txt:
  No cmake_minimum_required command is present.  A line of code such as

    cmake_minimum_required(VERSION 3.9)

  should be added at the top of the file.  The version specified may be lower
  if you wish to support older CMake versions for this project.  For more
  information run "cmake --help-policy CMP0000".
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Configuring incomplete, errors occurred!

How can I build clang-tidy or, alternatively, how can I install the latest version on macOS?

Solution

EDIT: this answer is out of date — the LLVM project has moved to a single git repository at https://github.com/llvm/llvm-project. See answers below for updated instructions.

clang-tidy is intended to be built inside a checkout of llvm/clang, and depends on CMake macros from the llvm project. You should check out the llvm repo, then the clang repo inside llvm/tools/clang, then the clang-tools-extra repo inside llvm/tools/clang/tools/extra. Then you can run CMake on the top-level directory, and make clang-tidy should work.

If you're not interested in building it yourself, it looks like the Homebrew formula for LLVM also includes the extra tools: https://github.com/Homebrew/homebrew-core/blob/382d3defb5bc48ce2dccd17261be70c4ada9a124/Formula/llvm.rb#L181