Remove `MacroKind::ProcMacroStub`

Remove `MacroKind::ProcMacroStub`

It's internal to resolve and always results in `Res::Err` outside of resolve.
Instead put `DefKind::Fn`s themselves into the macro namespace, it's ok.

Proc macro stubs are items placed into macro namespase for functions that define proc macros.
https://github.com/rust-lang/rust/pull/52383

The rustdoc test is changed because the old test didn't actually reproduce the ICE it was supposed to reproduce.