Unify labels pointing at the same span in move errors when possible

[estebank]

In tests/ui/moves/invalid-suggestions-destructuring-assignment-drop.stderr (and other cases) we currently output

error[E0509]: cannot move out of type `Thing`, which implements the `Drop` trait
  --> $DIR/invalid-suggestions-destructuring-assignment-drop.rs:12:34
   |
LL |     Thing(*&mut String::new()) = Thing(String::new());
   |           -------------------    ^^^^^^^^^^^^^^^^^^^^ cannot move out of here
   |           |
   |           data moved here
   |           move occurs because the place has type `String`, which does not implement the `Copy` trait

but ideally we should unify labels whenever possible. In this case, it should be

error[E0509]: cannot move out of type `Thing`, which implements the `Drop` trait
  --> $DIR/invalid-suggestions-destructuring-assignment-drop.rs:12:34
   |
LL |     Thing(*&mut String::new()) = Thing(String::new());
   |           -------------------    ^^^^^^^^^^^^^^^^^^^^ cannot move out of here
   |           |
   |           data moved here because the place has type `String`, which does not implement the `Copy` trait

[arferreira]

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