Flip assist for `+` in trait bounds

[Veykril]

We currently have a very nice flip comma assist which flips the left and right hand syntax of a comma, we should extend that to be applicable for trait bounds, ie dyn Foo + 'bar should allow flipping on the plus to get dyn 'bar + Foo

[evertedsphere]

I'll give this a shot.

@rustbot claim

[evertedsphere]

It looks like there already is a flip_trait_bound assist, it's just missing a test that checks that it works with trait objects.

[Veykril]

Right we do have that, hmm wonder why it didn't work when I tried it then 🤔