Add warn(unreachable_pub) to rustc_mir_dataflow.

compiler/rustc_mir_dataflow/src/lib.rs

@@ -5,6 +5,7 @@
 #![feature(exact_size_is_empty)]
 #![feature(let_chains)]
 #![feature(try_blocks)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use rustc_middle::ty;