Better classify what LLVM subproject is causing problems

[kwk]

When we still had standalone builds it was easy to point your finger at an LLVM sub-project that broke a build. We would assign a label such as project/llvm or project/clang. At the moment we only assign project/llvm all the time. It would be nice if more specific labels can be added from a path of a broken compilation or test. For starters, simply taking the first bit of a path should give us the project.