Bless tests, add comments

tests/ui/async-await/async-borrowck-escaping-closure-error.rs

@@ -4,7 +4,8 @@
 fn foo() -> Box<dyn std::future::Future<Output = u32>> {
     let x = 0u32;
     Box::new((async || x)())
-    //~^ ERROR closure may outlive the current function, but it borrows `x`, which is owned by the current function
+    //~^ ERROR cannot return value referencing local variable `x`
+    //~| ERROR cannot return value referencing temporary value
 }
 
 fn main() {