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.

src/test/rustdoc/proc-macro.rs

@@ -7,7 +7,7 @@
 // @has some_macros/index.html
 // @has - '//a/[@href="attr.some_proc_attr.html"]' 'some_proc_attr'
 
-//! include a link to [some_proc_attr] to make sure it works.
+//! include a link to [some_proc_macro] to make sure it works.
 
 extern crate proc_macro;