Switch to upstream llvm bazel config

Recently bazel support landed upstream in the llvm repo here: https://github.com/llvm/llvm-project/tree/main/utils/bazel

This support is technically best effort, but I think that's not significantly different from using rules_foreign_cc to include it today. I think switching to this would improve the build experience on envoy's side, and improve the stability of those upstream configs for the community.