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/lower_intrinsics.rs

@@ -7,7 +7,7 @@ use rustc_span::symbol::sym;
 
 use crate::take_array;
 
-pub struct LowerIntrinsics;
+pub(super) struct LowerIntrinsics;
 
 impl<'tcx> crate::MirPass<'tcx> for LowerIntrinsics {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {