Introduce the (WIP) THIR unsafety checker

compiler/rustc_interface/src/tests.rs

@@ -735,6 +735,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(symbol_mangling_version, Some(SymbolManglingVersion::V0));
     tracked!(teach, true);
     tracked!(thinlto, Some(true));
+    tracked!(thir_unsafeck, true);
     tracked!(tune_cpu, Some(String::from("abc")));
     tracked!(tls_model, Some(TlsModel::GeneralDynamic));
     tracked!(trap_unreachable, Some(false));