Create a valid Res in external_path()

The order of the `where` bounds on auto trait impls changed because
rustdoc currently sorts auto trait `where` bounds based on the `Debug`
output for the bound. Now that the bounds have an actual `Res`, they are
being unintentionally sorted by their `DefId` rather than their path.
So, I had to update a test for the change in ordering of the rendered
bounds.

src/test/rustdoc/synthetic_auto/no-redundancy.rs

@@ -10,7 +10,7 @@ where
 
 // @has no_redundancy/struct.Outer.html
 // @has - '//*[@id="synthetic-implementations-list"]//*[@class="impl has-srclink"]//h3[@class="code-header in-band"]' \
-// "impl<T> Send for Outer<T> where T: Copy + Send"
+// "impl<T> Send for Outer<T> where T: Send + Copy"
 pub struct Outer<T> {
     inner_field: Inner<T>,
 }