[llvm] updated buildkite pipeline generation (PR #65574)

[Louis Dionne via llvm-commits]

ldionne wrote:

I think it's fine to add `clang` to the set of projects in `LLVM_ENABLE_PROJECTS` for dependency purposes, but we could avoid calling `check-clang`. I guess it's moot cause we're using `check-all` right now.

We don't check `flang` in the Clang pipeline because the Clang pipeline's primary responsibility should be to test Clang, not other downstream projects.

https://github.com/llvm/llvm-project/pull/65574