[llvm] [GitHub] Bump clang-tidy version to 21th and add sccache in CI (PR #160551)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Isolating just the version bump into this change would probably be a good idea. That is straight forward and can just land.

Caching + container changes are going to require more discussion.

https://github.com/llvm/llvm-project/pull/160551