Add ccls cache to gitignore (#31437)
Summary:
`ccls` [puts a cache](https://github.com/MaskRay/ccls/wiki/Customization#cachedirectory) in the working directory by default, this PR adds it to gitignore so git doesn't pick it up
](https://our.intern.facebook.com/intern/diff/19165007/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31437
Pulled By: driazati
Differential Revision: D19165007
fbshipit-source-id: 41012eb0ece2df60b8566d7929710b154c38ee66
