I followed this page (Building Clang and Working with the Code, On Unix-like Systems) to download and build the clang source tree. The source tree looks just like this one. When building cppinsights inside clang (as mentioned in Readme.md) I encountered the following problems:

1. The path llvm/tools/clang/tools/extra does not exist. find . -name extra gives only ./llvm/test/tools/llvm-cov/Inputs/sources_specified/extra. I found the directory called clang-tools-extra which seems to be the right place since it contains other tools like clang-tidy and clang-query. Then I cloned cppinsights here and added add_subdirectory(cppinsights) into clang-tools-extra/CMakeLists.txt.
2. cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -G "Unix Makefiles" ../llvm gives the following error:

CMake Error at cmake/modules/LLVM-Config.cmake:106 (get_target_property):
  get_target_property() called with non-existent target "LLVM_INCLUDE_DIR".
Call Stack (most recent call first):
  cmake/modules/LLVM-Config.cmake:98 (explicit_llvm_config)
  /home/gkxx/install/llvm-project/clang-tools-extra/cppinsights/CMakeLists.txt:442 (llvm_config)

I have no idea how to fix this.