* Upgrade gtest for clang-tidy.
* Use CMake to install GTest instead of mv.
* Don't enforce clang-tidy to return 0 due to errors in thrust.
* Add a small test for tidy itself.
* Reformat.
* Basic script for using compilation database.
* Add `GENERATE_COMPILATION_DATABASE' to CMake.
* Rearrange CMakeLists.txt.
* Add basic python clang-tidy script.
* Remove modernize-use-auto.
* Add clang-tidy to Jenkins
* Refine logic for correct path detection
In Jenkins, the project root is of form /home/ubuntu/workspace/xgboost_PR-XXXX
* Run clang-tidy in CUDA 9.2 container
* Use clang_tidy container
