Auto merge of #128792 - compiler-errors:foreign-sig, r=spastorino

Use `FnSig` instead of raw `FnDecl` for `ForeignItemKind::Fn`, fix ICE for `Fn` trait error on safe foreign fn

Let's use `hir::FnSig` instead of `hir::FnDecl + hir::Safety` for `ForeignItemKind::Fn`. This consolidates some handling code between normal fns and foreign fns.

Separetly, fix an ICE where we weren't handling `Fn` trait errors for safe foreign fns.

If perf is bad for the first commit, I can rework the ICE fix to not rely on it. But if perf is good, I prefer we fix and clean up things all at once 👍

r? spastorino

Fixes #128764

Author: bors