Function bootstrap::core::build_steps::compile::compiler_rt_for_profiler

fn compiler_rt_for_profiler(builder: &Builder<'_>) -> PathBuf

Tries to find LLVM’s compiler-rt source directory, for building library/profiler_builtins.

Normally it lives in the src/llvm-project submodule, but if we will be using a downloaded copy of CI LLVM, then we try to use the compiler-rt sources from there instead, which lets us avoid checking out the LLVM submodule.