Previously it would error out:

```
$ x check --dry-run
thread 'main' panicked at 'std::fs::read_to_string(ci_llvm.join("link-type.txt")) failed with No such file or directory (os error 2) ("CI llvm missing: /home/joshua/rustc3/build/tmp-dry-run/x86_64-unknown-linux-gnu/ci-llvm")', src/bootstrap/config.rs:795:33
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Build completed unsuccessfully in 0:00:10
```