Rollup merge of #148855 - ZuseZ4:autodiff-lto-error, r=bjorn3

Error if an autodiff user does not set lto=fat

Based on your feedback, I started to provide a nice error message for a lack of `lto=fat`, instead of us forcing it.

In a next step, we should replace  `RUSTFLAGS="-Zautodiff=Enable"` with another Cargo.toml setting, as discussed here: rust-lang/rust#147487 (comment)

As another improvement, we should also figure out why rlib builds do not properly obey the fat=lto setting.

```````@bjorn3```````

Author: Zalathar