Encode only MIR that can be used by other crates

Only reachable items might participate in the code generation in the
downstream crates. Omit redundant optimized MIR of unreachable items
from a crate metadata.

Additionally, include reachable closures in reachable set, so that
unreachable closures can be omitted on the same basis.

compiler/rustc_passes/src/reachable.rs

@@ -90,6 +90,10 @@ impl<'tcx> Visitor<'tcx> for ReachableContext<'tcx> {
                 .typeck_results()
                 .type_dependent_def(expr.hir_id)
                 .map(|(kind, def_id)| Res::Def(kind, def_id)),
+            hir::ExprKind::Closure(&hir::Closure { def_id, .. }) => {
+                self.reachable_symbols.insert(def_id);
+                None
+            }
             _ => None,
         };