Rollup merge of #140000 - EnzymeAD:autodiff-check-builds, r=onur-ozkan

skip llvm-config in autodiff check builds, when its unavailable

As you suggested, this indeed fixes `./x.py check` builds when autodiff is enabled.

r? ```@onur-ozkan```

closes #139936

Tracking:

- rust-lang/rust#124509

Author: ChrisDenton