Set unaligned_references lint to deny in rustc_data_structures

To detect misuse of private packed field in `PackedFingerprint`.

compiler/rustc_data_structures/src/lib.rs

@@ -31,6 +31,7 @@
 #![feature(once_cell)]
 #![feature(maybe_uninit_uninit_array)]
 #![allow(rustc::default_hash_types)]
+#![deny(unaligned_references)]
 
 #[macro_use]
 extern crate tracing;