Reduce visibilities, and add warn(unreachable_pub).

Lots of unnecessary `pub`s in this crate. Most are downgraded to
`pub(super)`, though some don't need any visibility.

compiler/rustc_mir_transform/src/lint.rs

@@ -13,7 +13,7 @@ use rustc_mir_dataflow::impls::{MaybeStorageDead, MaybeStorageLive};
 use rustc_mir_dataflow::storage::always_storage_live_locals;
 use rustc_mir_dataflow::{Analysis, ResultsCursor};
 
-pub fn lint_body<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, when: String) {
+pub(super) fn lint_body<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, when: String) {
     let always_live_locals = &always_storage_live_locals(body);
 
     let maybe_storage_live = MaybeStorageLive::new(Cow::Borrowed(always_live_locals))