add config to explicitely test rustc with autodiff/enzyme disabled

Author: ZuseZ4

Diff for: src/tools/compiletest/src/command-list.rs

@@ -125,6 +125,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "needs-dlltool",
     "needs-dynamic-linking",
     "needs-enzyme",
+    "needs-enzyme-disabled",
     "needs-force-clang-based-tests",
     "needs-git-hash",
     "needs-llvm-components",

Diff for: src/tools/compiletest/src/header/needs.rs

@@ -84,6 +84,11 @@ pub(super) fn handle_needs(
             condition: config.has_enzyme,
             ignore_reason: "ignored when LLVM Enzyme is disabled",
         },
+        Need {
+            name: "needs-enzyme-disabled",
+            condition: !config.has_enzyme,
+            ignore_reason: "ignored when LLVM Enzyme is enabled",
+        },
         Need {
             name: "needs-run-enabled",
             condition: config.run_enabled(),