Add separate feature gate for async fn track caller

This patch adds a feature gate `async_fn_track_caller` that is separate from `closure_track_caller`. This is to allow enabling `async_fn_track_caller` separately.

Fixes #110009

compiler/rustc_span/src/symbol.rs

@@ -400,6 +400,7 @@ symbols! {
         async_await,
         async_closure,
         async_fn_in_trait,
+        async_fn_track_caller,
         atomic,
         atomic_mod,
         atomics,