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.

src/test/ui/async-await/feature-async-closure.stderr

@@ -6,6 +6,7 @@ LL |     let _ = async || {};
    |
    = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
    = help: add `#![feature(async_closure)]` to the crate attributes to enable
+   = help: to use an async block, remove the `||`: `async {`
 
 error: aborting due to previous error
 

src/test/ui/parser/block-no-opening-brace.rs

@@ -26,6 +26,6 @@ fn f4() {
     }
 
 fn f5() {
-    async //~ ERROR async closures are unstable
+    async
         let x = 0; //~ ERROR expected one of `move`, `|`, or `||`, found keyword `let`
     }

src/test/ui/parser/block-no-opening-brace.stderr

@@ -39,15 +39,5 @@ LL |     async
 LL |         let x = 0;
    |         ^^^ unexpected token
 
-error[E0658]: async closures are unstable
-  --> $DIR/block-no-opening-brace.rs:29:5
-   |
-LL |     async
-   |     ^^^^^
-   |
-   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
-   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+error: aborting due to 5 previous errors
 
-error: aborting due to 6 previous errors
-
-For more information about this error, try `rustc --explain E0658`.