Function rustc_llvm_env

fn rustc_llvm_env(
    builder: &Builder<'_>,
    cargo: &mut Cargo,
    target: TargetSelection,
)

Pass down configuration from the LLVM build into the build of rustc_llvm and rustc_codegen_llvm.

Note that this has the side-effect of building LLVM, which is sometimes unwanted (e.g. for check builds).