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