[llvm] [Github] Bump clang-format to 21.1.0 (PR #155660)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Should we wait until https://github.com/llvm/llvm-project/pull/155466 is merged?

That's only needed for the reproduction instructions. We're not actually using it in the CI. The way the git history gets setup in the CI is quite a bit different than it typically is locally due to how Github handles things.

https://github.com/llvm/llvm-project/pull/155660