Suggest expressions that look like const generic arguments should be enclosed in brackets

Co-Authored-By: Esteban Kuber <github@kuber.com.ar>

compiler/rustc_parse/src/parser/mod.rs

@@ -36,6 +36,7 @@ bitflags::bitflags! {
     struct Restrictions: u8 {
         const STMT_EXPR         = 1 << 0;
         const NO_STRUCT_LITERAL = 1 << 1;
+        const CONST_EXPR        = 1 << 2;
     }
 }