Fix async-std at the price of breaking half the test suite

- Don't mark impl trait as an error

src/test/rustdoc-ui/error-in-impl-trait/trait-alias.rs

@@ -1,3 +1,4 @@
+// check-pass
 #![feature(type_alias_impl_trait)]
 
 pub trait ValidTrait {}
@@ -6,5 +7,4 @@ type ImplTrait = impl ValidTrait;
 /// This returns impl trait, but using a type alias
 pub fn h() -> ImplTrait {
     error::_in::impl_trait::alias()
-    //~^ ERROR failed to resolve
 }