Use PredicateObligations instead of Predicates

Keep more information about trait binding failures.

src/test/ui/unsized/unsized-enum.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/unsized-enum.rs:6:36
    |
 LL | enum Foo<U> { FooSome(U), FooNone }
-   | ----------- required by `Foo`
+   |      --- - required by this bound in `Foo`
 LL | fn foo1<T>() { not_sized::<Foo<T>>() } // Hunky dory.
 LL | fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() }
    |         -                          ^^^^^^ doesn't have a size known at compile-time

src/test/ui/unsized/unsized-inherent-impl-self-type.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized-inherent-impl-self-type.rs:7:17
    |
 LL | struct S5<Y>(Y);
-   | ---------------- required by `S5`
+   |        -- - required by this bound in `S5`
 LL | 
 LL | impl<X: ?Sized> S5<X> {
    |      -          ^^^^^ doesn't have a size known at compile-time

src/test/ui/unsized/unsized-struct.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/unsized-struct.rs:6:36
    |
 LL | struct Foo<T> { data: T }
-   | ------------- required by `Foo`
+   |        --- - required by this bound in `Foo`
 LL | fn foo1<T>() { not_sized::<Foo<T>>() } // Hunky dory.
 LL | fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() }
    |         -                          ^^^^^^ doesn't have a size known at compile-time

src/test/ui/unsized/unsized-trait-impl-self-type.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized-trait-impl-self-type.rs:10:17
    |
 LL | struct S5<Y>(Y);
-   | ---------------- required by `S5`
+   |        -- - required by this bound in `S5`
 LL | 
 LL | impl<X: ?Sized> T3<X> for S5<X> {
    |      -          ^^^^^ doesn't have a size known at compile-time