inconsistent lint flag args between normal and post-rustfix compiles in UI tests

[zackmdavis]

UI tests run with-A unused, but UI tests with the run-rustfix directive go on to apply suggestions and then check that there are no warnings, without passing any lint flags. The inconsistency is a papercut–footgun for UI test authors. We should do exactly one of—

• Pass -A unused for the rustfix-succeeded check, too; or
• Stop passing -A unused for UI tests.
  • The present author gave an argument for this in reconsider magic -A unused for UI tests? #43896, but the reception seemed unfavorable (one 👎 react and one negatively-disposed comment). If it turned out that there was support for this after all, we would want someone to edit existing tests to not produce spurious unused warnings, which would be a very large PR.

[Mark-Simulacrum]

It does seem like passing -A unused to rustfix would be a good change to make here, and at a cursory look at the code it seems like we do that today? But if I'm wrong then we can, of course, reopen.