Rollup merge of #136419 - EnzymeAD:autodiff-tests, r=onur-ozkan,jieyouxu

adding autodiff tests

I'd like to get started with upstreaming some tests, even though I'm still waiting for an answer on how to best integrate the enzyme pass. Can we therefore temporarily support the -Z llvm-plugins here without too much effort? And in that case, how would that work? I saw you can do remapping, e.g. `rust-src-base`, but I don't think that will give me the path to libEnzyme.so. Do you have another suggestion?

Other than that this test simply checks that the derivative of `x*x` is `2.0 * x`, which in this case is computed as
`%0 = fadd fast double %x.0.val, %x.0.val`
(I'll add a few more tests and move it to an autodiff folder if we can use the -Z flag)

r? ``@jieyouxu``

Locally at least `-Zllvm-plugins=${PWD}/build/x86_64-unknown-linux-gnu/enzyme/build/Enzyme/libEnzyme-19.so` seems to work if I copy the command I get from x.py test and run it manually. However, running x.py test itself fails.

Tracking:

- https://github.com/rust-lang/rust/issues/124509

Zulip discussion: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Enzyme.20build.20changes

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -914,8 +914,6 @@ fn autodiff_attrs(tcx: TyCtxt<'_>, id: DefId) -> Option<AutoDiffAttrs> {
     let mode = match mode.as_str() {
         "Forward" => DiffMode::Forward,
         "Reverse" => DiffMode::Reverse,
-        "ForwardFirst" => DiffMode::ForwardFirst,
-        "ReverseFirst" => DiffMode::ReverseFirst,
         _ => {
             span_bug!(mode.span, "rustc_autodiff attribute contains invalid mode");
         }