Split `autodiff` into `autodiff_forward` and `autodiff_reverse`

Pending fix.
```
error: cannot find a built-in macro with name `autodiff_forward`
    --> library\core\src\macros\mod.rs:1542:5
     |
1542 | /     pub macro autodiff_forward($item:item) {
1543 | |         /* compiler built-in */
1544 | |     }
     | |_____^

error: cannot find a built-in macro with name `autodiff_reverse`
    --> library\core\src\macros\mod.rs:1549:5
     |
1549 | /     pub macro autodiff_reverse($item:item) {
1550 | |         /* compiler built-in */
1551 | |     }
     | |_____^

error: could not compile `core` (lib) due to 2 previous errors
```

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/rustc_builtin_macros/src/lib.rs b/compiler/rustc_builtin_macros/src/lib.rs
index 9cd4d17059a..a89b3642f7e 100644
--- a/compiler/rustc_builtin_macros/src/lib.rs
+++ b/compiler/rustc_builtin_macros/src/lib.rs
@@ -112,7 +112,8 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
 
     register_attr! {
         alloc_error_handler: alloc_error_handler::expand,
-        autodiff: autodiff::expand,
+        autodiff_forward: autodiff::expand_forward,
+        autodiff_reverse: autodiff::expand_reverse,
         bench: test::expand_bench,
         cfg_accessible: cfg_accessible::Expander,
         cfg_eval: cfg_eval::expand,