autodiff rlib handling
As I learned recently, we now apparently support rlib builds already in some cases.
With the last hint from saethlin this seems to now cover all cases. To be sure I'll add a few more testcases before I mark it as ready.
Once this PR lands, we should to the best of my knowledge, support autodiff in almost code locations, only vtable/dyn ptr remain unsupported for now.
r? ghost
closes: https://github.com/rust-lang/rust/issues/148856
closes: https://github.com/rust-lang/rust/issues/137520
710720a827