Rename -Zno_parallel_llvm -> -Zno_parallel_backend

compiler/rustc_interface/src/tests.rs

@@ -685,7 +685,7 @@ fn test_unstable_options_tracking_hash() {
     untracked!(nll_facts, true);
     untracked!(no_analysis, true);
     untracked!(no_leak_check, true);
-    untracked!(no_parallel_llvm, true);
+    untracked!(no_parallel_backend, true);
     untracked!(parse_only, true);
     // `pre_link_arg` is omitted because it just forwards to `pre_link_args`.
     untracked!(pre_link_args, vec![String::from("abc"), String::from("def")]);