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

[Louis Dionne via llvm-commits]

ldionne wrote:

@metaflow The reason for not adding clang to the monolithic pipeline is that clang now has its own pipeline like libc++. After your patch we will be building and testing clang twice for no reason. Can you remove it again?

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