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.

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -77,7 +77,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
         ty: Ty<'tcx>,
         orig_env: ty::ParamEnv<'tcx>,
         trait_did: DefId,
-        auto_trait_callback: impl Fn(&InferCtxt<'_, 'tcx>, AutoTraitInfo<'tcx>) -> A,
+        mut auto_trait_callback: impl FnMut(&InferCtxt<'_, 'tcx>, AutoTraitInfo<'tcx>) -> A,
     ) -> AutoTraitResult<A> {
         let tcx = self.tcx;