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/unreachable_enum_branching.rs

@@ -12,7 +12,7 @@ use rustc_middle::ty::{Ty, TyCtxt};
 use rustc_target::abi::{Abi, Variants};
 use tracing::trace;
 
-pub struct UnreachableEnumBranching;
+pub(super) struct UnreachableEnumBranching;
 
 fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {
     if let TerminatorKind::SwitchInt { discr: Operand::Move(p), .. } = terminator {