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/function_item_references.rs

@@ -11,7 +11,7 @@ use rustc_target::spec::abi::Abi;
 
 use crate::errors;
 
-pub struct FunctionItemReferences;
+pub(super) struct FunctionItemReferences;
 
 impl<'tcx> crate::MirLint<'tcx> for FunctionItemReferences {
     fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {