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/libsyntax/ext/base.rs

@@ -527,8 +527,6 @@ pub enum MacroKind {
     Attr,
     /// A derive attribute macro - #[derive(Foo)]
     Derive,
-    /// A view of a procedural macro from the same crate that defines it.
-    ProcMacroStub,
 }
 
 impl MacroKind {
@@ -537,7 +535,6 @@ impl MacroKind {
             MacroKind::Bang => "macro",
             MacroKind::Attr => "attribute macro",
             MacroKind::Derive => "derive macro",
-            MacroKind::ProcMacroStub => "crate-local procedural macro",
         }
     }