Add test for `?const` and `?` on the same bound

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/with-maybe-sized.rs

+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
+
+struct S<T: ?const ?Sized>(std::marker::PhantomData<T>);
+//~^ ERROR `?const` and `?` are mutually exclusive
+//~| ERROR `?const` on trait bounds is not yet implemented
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-trait-bound-opt-out/with-maybe-sized.stderr

+error: `?const` and `?` are mutually exclusive
+  --> $DIR/with-maybe-sized.rs:4:13
+   |
+LL | struct S<T: ?const ?Sized>(std::marker::PhantomData<T>);
+   |             ^^^^^^^^^^^^^
+
+error: `?const` on trait bounds is not yet implemented
+  --> $DIR/with-maybe-sized.rs:4:13
+   |
+LL | struct S<T: ?const ?Sized>(std::marker::PhantomData<T>);
+   |             ^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+