Implement the unsafe-fields RFC.

Co-Authored-By: Jacob Pratt <jacob@jhpratt.dev>

compiler/rustc_hir/src/hir.rs

@@ -3177,6 +3177,7 @@ pub struct FieldDef<'hir> {
     pub hir_id: HirId,
     pub def_id: LocalDefId,
     pub ty: &'hir Ty<'hir>,
+    pub safety: Safety,
 }
 
 impl FieldDef<'_> {