Obazl_llvm - bazel module(s) for llvm

I’ve got a Proof-of-Concept Bazel module with a (rudimentary) new toolchain and modularized SDKs. It uses implementation techniques made possible by Bazel modules so it looks quite a bit different than existing Bazel stuff. In particular it makes it quite easy to integrate a local llvm installation (binary distro or source build) if you just want to use an llvm or clang SDK. OTOH at this point you might not want to mess with it unless you’re interested in Bazel development - the toolchain in particular is pretty minimal and needs the help of clang experts, which I am not. Feedback welcome.