Don't add deny(unreachable_pub) to rustc_baked_icu.

But explain why.

Author: nnethercote

compiler/rustc_baked_icu_data/src/lib.rs

@@ -23,6 +23,7 @@
 // tidy-alphabetical-start
 #![allow(elided_lifetimes_in_paths)]
 #![allow(internal_features)]
+// #![deny(unreachable_pub)] // don't use because this crate is mostly generated code
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
 // tidy-alphabetical-end