Fix as_underscore to only suggest when it's suggestable

[teofr]

Have as_underscore only make a suggestion when the type is suggestable.

changelog: [as_underscore]: Don't suggest a fix if the type is not suggestable

Fixes #15282

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[teofr]

Some notes:

• I minimized the example from the issue, since a lot of it wasn't part of the bug
• I separated the tests into two, since I couldn't find a way to tell cargo to expect some lints to be unfixable, only at the file level

[Alexendoo]

👍

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