rustc: Remove #[rustc_synthetic]

This function parameter attribute was introduced in https://github.com/rust-lang/rust/pull/44866 as an intermediate step in implementing `impl Trait`, it's not necessary or used anywhere by itself.

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -290,9 +290,10 @@ fn suggest_restriction(
     } else {
         // Trivial case: `T` needs an extra bound: `T: Bound`.
         let (sp, suggestion) = match (
-            generics.params.iter().find(|p| {
-                !matches!(p.kind, hir::GenericParamKind::Type { synthetic: Some(_), .. })
-            }),
+            generics
+                .params
+                .iter()
+                .find(|p| !matches!(p.kind, hir::GenericParamKind::Type { synthetic: true, .. })),
             super_traits,
         ) {
             (_, None) => predicate_constraint(