Single commit implementing the enzyme/autodiff frontend

Co-authored-by: Lorenz Schmidt <bytesnake@mailbox.org>

compiler/rustc_span/src/symbol.rs

@@ -481,6 +481,8 @@ symbols! {
         audit_that,
         augmented_assignments,
         auto_traits,
+        autodiff,
+        autodiff_fallback,
         automatically_derived,
         avx,
         avx512_target_feature,
@@ -544,6 +546,7 @@ symbols! {
         cfg_accessible,
         cfg_attr,
         cfg_attr_multi,
+        cfg_autodiff_fallback,
         cfg_boolean_literals,
         cfg_doctest,
         cfg_eval,
@@ -998,6 +1001,7 @@ symbols! {
         hashset_iter_ty,
         hexagon_target_feature,
         hidden,
+        hint,
         homogeneous_aggregate,
         host,
         html_favicon_url,
@@ -1650,6 +1654,7 @@ symbols! {
         rustc_allow_incoherent_impl,
         rustc_allowed_through_unstable_modules,
         rustc_attrs,
+        rustc_autodiff,
         rustc_box,
         rustc_builtin_macro,
         rustc_capture_analysis,