Make Clean take &mut DocContext

- Take `FnMut` in `rustc_trait_selection::find_auto_trait_generics`
- Take `&mut DocContext` in most of `clean`
- Collect the iterator in auto_trait_impls instead of iterating lazily; the lifetimes were really bad.
- Changes `fn sess` to properly return a borrow with the lifetime of `'tcx`, not the mutable borrow.

src/librustdoc/html/render/mod.rs

@@ -181,7 +181,7 @@ impl<'tcx> Context<'tcx> {
         self.shared.tcx
     }
 
-    fn sess(&self) -> &Session {
+    fn sess(&self) -> &'tcx Session {
         &self.shared.tcx.sess
     }
 }