Fix missing lifetimes diagnostics after #83759
In #83759 while rebasing I didn't realize there was a new function for suggesting to add lifetime arguments. It relied on some invariants, namely that if a generic type/trait has angle brackets then it must have some generic argument, which is now no longer true. This PR updates that function to handle the new invariants.
This also adds a new regression test but I'm not sure if that's the correct place for it.
Fixes#85347
67d5435695