Structured suggestion for "missing feature intrinsic"

```
error: `size_of_val` is not yet stable as a const intrinsic
  --> $DIR/const-unstable-intrinsic.rs:17:9
   |
LL |         unstable_intrinsic::size_of_val(&x);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add `#![feature(unstable)]` to the crate attributes to enable
help: add `#![feature(unstable)]` to the crate attributes to enable
   |
LL + #![feature("unstable")]
   |
```

compiler/rustc_const_eval/src/errors.rs

@@ -123,12 +123,19 @@ pub(crate) struct UnstableConstFn {
 
 #[derive(Diagnostic)]
 #[diag(const_eval_unstable_intrinsic)]
-#[help]
 pub(crate) struct UnstableIntrinsic {
     #[primary_span]
     pub span: Span,
     pub name: Symbol,
     pub feature: Symbol,
+    #[suggestion(
+        const_eval_unstable_intrinsic_suggestion,
+        code = "#![feature({feature})]\n",
+        applicability = "machine-applicable"
+    )]
+    pub suggestion: Option<Span>,
+    #[help(const_eval_unstable_intrinsic_suggestion)]
+    pub help: bool,
 }
 
 #[derive(Diagnostic)]