Disallow reference to static mut for expressions

Add `E0796` error code.
Add `static_mut_ref` lint.

This is the idea for the 2024 edition.

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -66,6 +66,7 @@ mod check;
 mod compare_impl_item;
 pub mod dropck;
 mod entry;
+mod errs;
 pub mod intrinsic;
 pub mod intrinsicck;
 mod region;