enzyme/autodiff is compatible with download-ci=true

[ZuseZ4]

To my surprise autodiff just works out of the box with download-ci=true. Thanks to all the bootstrap people who over the past helped me to link properly against the sysroot LLVM, which seems to pay off here.
That also helps with enabling Rust in Enzyme CI, since CCache for some reason doesn't seem to cache LLVM correctly on their runners.

I verified that this works with
./configure --release-channel=nightly --enable-llvm-enzyme --enable-option-checking --disable-docs --enable-llvm-assertions --set llvm.download-ci-llvm=true. However, shouldn't download-ci-llvm already be the default? Why do I still have to manually set it with this pr?

I tested it afterwards with
./x.py test --stage 1 tests/codegen-llvm/autodiff

r? bootstrap

closes #147535

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang