std::autodiff should work with download-llvm

[ZuseZ4]

Summary

The current build instructions for rustc with std::autodiff enabled are given here:
https://rustc-dev-guide.rust-lang.org/autodiff/installation.html#build-instructions
Especially

./configure --enable-llvm-link-shared --enable-llvm-plugins --enable-llvm-enzyme --release-channel=nightly --enable-llvm-assertions --enable-clang --enable-lld --enable-option-checking --enable-ninja --disable-docs

We can already drop the --enable-llvm-plugins, as well as --enable-llvm-assertions --enable-clang --enable-lld
They are for convenience, but not strictly needed.

Command used

./x build --stage 1 library
./x test --stage 1 tests/codegen-llvm/autodiff
./x test --stage 1 tests/pretty/autodiff
./x test --stage 1 tests/ui/autodiff
./x test --stage 1 tests/ui/feature-gates/feature-gate-autodiff.rs

Expected behaviour

It should build rustc and pass all autodiff tests

Actual behaviour

The last time I tested the build failed. Building Enzyme in bootstrap requires the path to LLVM, and that path is different when we build LLVM vs when we download it.
We should handle both cases.

Bootstrap configuration (bootstrap.toml)

<config>

Operating system

HEAD

Additional context

Build Log

[ZuseZ4]

Current status: We experimented with this, but the issue runs a bit deeper than we first thought.
After adding the missing cmake files to the llvm component against which we build Enzyme, we encountered the issue that Enzyme also requires additional llvm archives (e.g. for demangling). Shipping those is a much bigger problem than just adding a few cmake files.

I guess if someone wants to push this, a good way to experiment would be to look at the PRs linked above and figuring out what artifacts exactly are required, if we potentially already build them in CI, and if their need on the Enzyme side could be removed.

The problem is that our CI dist runners, which build LLVM, are already our (by far) slowest CI runners and hold back the speed at which we merge PRs. A bit of extra time for them to build Enzyme isn't great, but at least it will enable every nightly user to use autodiff.
However, there are a lot less rustc-Enzyme devs than users, so making those CI runners slower for everyone just for us ~5 contributors to benefit would be a bit much, so it would be good to have a full list of what libraries exactly we need.