std::autodiff should work with download-llvm

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

Start with the autodiff build instructions in rustc-dev-guide/autodiff/installation.html and the bootstrap path used to build Enzyme. Reproduce with download-llvm using the listed ./x build and ./x test commands, then trace the LLVM path passed to Enzyme's CMake invocation. Done means rustc builds and all listed autodiff tests pass.