syntax: add a missing span rewrite in fold.

This was leaving Decls without the new spans; this is a minor change,
since literally nothing reads in the code base reads the span of a Decl
itself, always just its contents.

src/libsyntax/fold.rs

@@ -134,7 +134,7 @@ pub trait Folder {
         node.move_iter().map(|node| {
             @Spanned {
                 node: node,
-                span: d.span,
+                span: self.new_span(d.span),
             }
         }).collect()
     }