Make -Z new-llvm-pass-manager an Option<bool>

To allow it to have an LLVM version dependent default.

compiler/rustc_interface/src/tests.rs

@@ -709,7 +709,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(mir_emit_retag, true);
     tracked!(mir_opt_level, Some(4));
     tracked!(mutable_noalias, Some(true));
-    tracked!(new_llvm_pass_manager, true);
+    tracked!(new_llvm_pass_manager, Some(true));
     tracked!(no_codegen, true);
     tracked!(no_generate_arange_section, true);
     tracked!(no_link, true);