Rollup merge of #145429 - bjorn3:codegen_fn_attrs_improvements, r=jdonszelmann

Couple of codegen_fn_attrs improvements

As noted in rust-lang/rust#144678 (comment) here is no need to keep link_name and export_name separate, which the third commit fixes by merging them. The second commit removes some dead code and the first commit merges two ifs with equivalent conditions. The last commit is an unrelated change which removes an unused `feature(autodiff)`.

Author: jieyouxu