Catch forward declarations in default type params at AST conversion.

src/test/compile-fail/generic-type-params-forward-mention.rs

@@ -12,8 +12,7 @@
 
 // Ensure that we get an error and not an ICE for this problematic case.
 struct Foo<T = Option<U>, U = bool>;
-
+//~^ ERROR type parameters with a default cannot use forward declared identifiers
 fn main() {
     let x: Foo;
-    //~^ ERROR missing type param `U` in the substitution of `std::option::Option<U>`
 }