Use PredicateObligations instead of Predicates

Keep more information about trait binding failures.

src/test/ui/bad/bad-sized.stderr

@@ -14,10 +14,14 @@ error[E0277]: the size for values of type `dyn Trait` cannot be known at compila
    |
 LL |     let x: Vec<dyn Trait + Sized> = Vec::new();
    |            ^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   | 
+  ::: $SRC_DIR/liballoc/vec.rs:LL:COL
+   |
+LL | pub struct Vec<T> {
+   |                - required by this bound in `std::vec::Vec`
    |
    = help: the trait `std::marker::Sized` is not implemented for `dyn Trait`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
-   = note: required by `std::vec::Vec`
 
 error[E0277]: the size for values of type `dyn Trait` cannot be known at compilation time
   --> $DIR/bad-sized.rs:4:37