Function prebuilt_llvm_config

pub fn prebuilt_llvm_config(
    builder: &Builder<'_>,
    target: TargetSelection,
    handle_submodule_when_needed: bool,
) -> LlvmBuildStatus

This returns whether we’ve already previously built LLVM.

It’s used to avoid busting caches during x.py check – if we’ve already built LLVM, it’s fine for us to not try to avoid doing so.

This will return the llvm-config if it can get it (but it will not build it if not).