test building enzyme in CI
1) This PR fixes a significant compile-time regression, by only running the expensive autodiff pipeline, if the users pass the newly introduced Enable value to the `-Zautodiff=` flag. It updates the test(s) accordingly. It gives a nice error if users forget that.
2) It fixes macos support by explicitly linking against the Enzyme build folder. This doesn't cover CI macos yet.
3) It fixes the issue that setting ENZYME_RUNPASS was ignored by enzyme and in fact did not schedule enzyme's opt pass.
4) It also re-enables support for various other values for the autodiff flag, which were ignored since the refactor.
5) I merged some improvements to Enzyme core, which means we do not longer depend on LLVM being build with the Plugin Interface enabled.
6) Unrelated to other fixes, this changes `rustc_autodiff` to `EncodeCrossCrate::Yes`. It is not enough on it's own to enable usage of Enzyme in libraries, but it is for sure a piece of the fixes needed to get this to work.
try-job: x86_64-gnu
r? `@oli-obk`
Tracking:
- https://github.com/rust-lang/rust/issues/124509
8dac72bb1d