Factor out AttrsTarget flattening code.

This commit does the following.
- Pulls the code out of `AttrTokenStream::to_token_trees` into a new
  function `attrs_and_tokens_to_token_trees`.
- Simplifies `TokenStream::from_ast` by calling the new function. This
  is nicer than the old way, which created a temporary
  `AttrTokenStream` containing a single `AttrsTarget` (which required
  some cloning) just to call `to_token_trees` on it. (It is good to
  remove this use of `AttrsTarget` which isn't related to `cfg_attr`
  expansion.)

compiler/rustc_expand/src/config.rs

@@ -283,7 +283,10 @@ impl<'a> StripUnconfigured<'a> {
                 .flat_map(|item| self.process_cfg_attr(&self.expand_cfg_attr_item(cfg_attr, item)))
                 .collect()
         } else {
-            expanded_attrs.into_iter().map(|item| self.expand_cfg_attr_item(cfg_attr, item)).collect()
+            expanded_attrs
+                .into_iter()
+                .map(|item| self.expand_cfg_attr_item(cfg_attr, item))
+                .collect()
         }
     }