When suggesting trait bounds, use fully-qualified path

[estebank]

This side-steps issues of name clashes with imports or traits in the current scope.

error[E0277]: the trait bound `T: MyTrait` is not satisfied
  --> $DIR/bound-suggestions.rs:72:27
   |
LL |     fn method<T: MyTrait, W: super::Wrapper<T>>() {}
   |                              ^^^^^^^^^^^^^^^^^ the trait `MyTrait` is not implemented for `T`
   |
note: required by a bound in `Wrapper`
  --> $DIR/bound-suggestions.rs:66:18
   |
LL | trait Wrapper<T: MyTrait> {}
   |                  ^^^^^^^ required by this bound in `Wrapper`
help: consider further restricting type parameter `T` with trait `MyTrait`
   |
LL |     fn method<T: MyTrait + crate::MyTrait, W: super::Wrapper<T>>() {}
   |                          ++++++++++++++++

Ideally we'd lean on resolve to have the best name for a DefId on a given scope, but then we'd have to ferry the scope around for pretty much every diagnostic. This is the second best thing that does address the problem of "the suggestion is broken". This PR addresses #36184 on edition>=2018. For edition 2015 we'd have to suggest super::MyTrait.

[rustbot]

r? @BoxyUwU

rustbot has assigned @BoxyUwU.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer