Compute upvars lazily.

It can be computed from `tcx` on demand, instead of computing it eagerly
and passing it around.

compiler/rustc_borrowck/src/lib.rs

@@ -217,7 +217,6 @@ fn do_mir_borrowck<'tcx>(
         &mut flow_inits,
         &move_data,
         &borrow_set,
-        tcx.closure_captures(def),
         consumer_options,
     );