Emit fake borrows for all tests

I was incorrectly under the impression that this would only lead to
duplicates. See `mir-opt/match-arm-scope.rs` (upcomming commit) for a
case where we didn't emit a fake borrow of `items.1`.

src/librustc_mir/build/matches/mod.rs

@@ -870,7 +870,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             span,
             untested_candidates,
             join_block,
-            &mut None,
+            fake_borrows,
         )
     }