rustdoc: Fix local reexports of proc macros

Filter out `ProcMacroStub`s to avoid an ICE during cleaning.

Also add proc macros to `cache().paths` so it can generate links.

src/librustdoc/html/render.rs

@@ -1512,7 +1512,8 @@ impl DocFolder for Cache {
             clean::FunctionItem(..) | clean::ModuleItem(..) |
             clean::ForeignFunctionItem(..) | clean::ForeignStaticItem(..) |
             clean::ConstantItem(..) | clean::StaticItem(..) |
-            clean::UnionItem(..) | clean::ForeignTypeItem | clean::MacroItem(..)
+            clean::UnionItem(..) | clean::ForeignTypeItem |
+            clean::MacroItem(..) | clean::ProcMacroItem(..)
             if !self.stripped_mod => {
                 // Re-exported items mean that the same id can show up twice
                 // in the rustdoc ast that we're looking at. We know,