Fix wrong suggestion for async gen block

Author: GuillaumeGomez

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3376,9 +3376,12 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
 
         let (sugg_span, suggestion) = match tcx.sess.source_map().span_to_snippet(args_span) {
             Ok(string) => {
-                let coro_prefix = if string.starts_with("async") {
-                    // `async` is 5 chars long. Not using `.len()` to avoid the cast from `usize`
-                    // to `u32`.
+                let coro_prefix = if string.starts_with("async gen") {
+                    // `async gen` is 5 chars long. Not using `.len()` to avoid the cast from
+                    // `usize` to `u32`.
+                    Some(9)
+                } else if string.starts_with("async") {
+                    // `async` is 5 chars long.
                     Some(5)
                 } else if string.starts_with("gen") {
                     // `gen` is 3 chars long