Merge unboxed trait object error suggestion into regular dyn incompat error

[oli-obk]

Another hir-walker removed from the well-formed queries. This error was always a duplicate of another, but it was able to provide more information because it could invoke is_dyn_compatible without worrying about cycle errors. That's also the reason we can't put the error directly into hir_ty_lowering when lowering a dyn Trait within an associated item signature. So instead I packed it into the error handling of wf obligation checking.

[rustbot]

r? @wesleywiser

rustbot has assigned @wesleywiser.
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