Suggest async {} for async || {}

Fixes #76011

This adds support for adding help diagnostics to the feature gating checks and
then uses it for the async_closure gate to add the extra bit of help
information as described in the issue.

compiler/rustc_parse/src/parser/expr.rs

@@ -1554,10 +1554,6 @@ impl<'a> Parser<'a> {
         } else {
             Async::No
         };
-        if let Async::Yes { span, .. } = asyncness {
-            // Feature-gate `async ||` closures.
-            self.sess.gated_spans.gate(sym::async_closure, span);
-        }
 
         let capture_clause = self.parse_capture_clause();
         let decl = self.parse_fn_block_decl()?;
@@ -1574,6 +1570,11 @@ impl<'a> Parser<'a> {
             }
         };
 
+        if let Async::Yes { span, .. } = asyncness {
+            // Feature-gate `async ||` closures.
+            self.sess.gated_spans.gate(sym::async_closure, span);
+        }
+
         Ok(self.mk_expr(
             lo.to(body.span),
             ExprKind::Closure(capture_clause, asyncness, movability, decl, body, lo.to(decl_hi)),