clang-format don't run on master (#37058)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37058
We shouldn't add advisory checks to master, because PRs will get
reverted if they fail. This PR makes the following changes:
1. Factor out the binary fetch logic into `clang_format_utils.py`
2. Copypasta the canonical git integration from llvm and modify it to
use our binary fetcher. No more bikeshedding about how to integrate,
we just use the standard integration.
3. Change the CI job to run on pull-requests only and use
`git-clang-format`.
4. The original `clang_format.py` is now renamed `clang_format_all.py`
to reflect its purpose.
5. The pre-commit hook has been changed to use `git-clang-format`.
For pre-commit hook users: no changes required.
For others: add `tools/git-clang-format` to your PATH and you can do `git clang-format` to format your working tree.
Test Plan: Imported from OSS
Differential Revision: D21180893
Pulled By: suo
fbshipit-source-id: f8358fb7ce26f11585226aaac5ed89d379257bfb
