Port rustc_nonnull_optimization_guaranteed to the new attribute parser

tests/ui/feature-gates/feature-gate-rustc-attrs-1.rs

@@ -5,10 +5,12 @@
 //~| NOTE the `#[rustc_nonnull_optimization_guaranteed]` attribute is an internal implementation detail that will never be stable
 //~| NOTE  the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to document guaranteed niche optimizations in the standard library
 //~| NOTE the compiler does not even check whether the type indeed is being non-null-optimized; it is your responsibility to ensure that the attribute is only used on types that are optimized
-fn main() {}
+struct Foo {}
 
 #[rustc_variance]
 //~^ ERROR use of an internal attribute [E0658]
 //~| NOTE the `#[rustc_variance]` attribute is an internal implementation detail that will never be stable
 //~| NOTE the `#[rustc_variance]` attribute is used for rustc unit tests
 enum E {}
+
+fn main() {}