Rollup merge of #100734 - ComputerDruid:afit_feature, r=compiler-errors

Split out async_fn_in_trait into a separate feature

PR #101224 added support for async fn in trait desuraging behind the `return_position_impl_trait_in_trait` feature.

Split this out so that it's behind its own feature gate, since async fn in trait doesn't need to follow the same stabilization schedule.

compiler/rustc_span/src/symbol.rs

@@ -396,6 +396,7 @@ symbols! {
         assume_init,
         async_await,
         async_closure,
+        async_fn_in_trait,
         atomic,
         atomic_mod,
         atomics,