Add warn(unreachable_pub) to rustc_const_eval.

compiler/rustc_const_eval/src/lib.rs

@@ -14,6 +14,7 @@
 #![feature(trait_alias)]
 #![feature(try_blocks)]
 #![feature(yeet_expr)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub mod check_consts;