CI: install clang-tidy (#23518)
Summary:
Install clang-tidy (from LLVM 8) for the `clang_tidy` job.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23518
Differential Revision: D16549621
Pulled By: ezyang
fbshipit-source-id: b1d20641380cdfdb0589249770b98163528fa69f
