Represent ownership transfer in RustdocVisitor::visit

Previously visit could be called multiple times, but this is inaccurate,
as it deconstructs Visitor state.

src/librustdoc/core.rs

@@ -392,9 +392,9 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
             debug!("crate: {:?}", tcx.hir().krate());
 
             let mut krate = {
-                let mut v = RustdocVisitor::new(&ctxt);
+                let v = RustdocVisitor::new(&ctxt);
                 let module = v.visit(tcx.hir().krate());
-                (v, module).clean(&ctxt)
+                ((), module).clean(&ctxt)
             };
 
             fn report_deprecated_attr(name: &str, diag: &errors::Handler) {