syntax: Capture a TokenStream when parsing items

This is then later used by `proc_macro` to generate a new
`proc_macro::TokenTree` which preserves span information. Unfortunately this
isn't a bullet-proof approach as it doesn't handle the case when there's still
other attributes on the item, especially inner attributes.

Despite this the intention here is to solve the primary use case for procedural
attributes, attached to functions as outer attributes, likely bare. In this
situation we should be able to now yield a lossless stream of tokens to preserve
span information.

src/libsyntax_ext/deriving/generic/mod.rs

@@ -504,6 +504,7 @@ impl<'a> TraitDef<'a> {
                 defaultness: ast::Defaultness::Final,
                 attrs: Vec::new(),
                 node: ast::ImplItemKind::Type(type_def.to_ty(cx, self.span, type_ident, generics)),
+                tokens: None,
             }
         });
 
@@ -930,6 +931,7 @@ impl<'a> MethodDef<'a> {
                                                 decl: fn_decl,
                                             },
                                             body_block),
+            tokens: None,
         }
     }