emit unused_attributes lint for silly attributes

[zackmdavis]

We already have an UnusedAttributes late lint pass for attributes that don't get marked as having been used somewhere, but recent discussion has touched on the idea of also directly (during normal complication, not as a lint pass) issuing the lint for attributes that don't make much sense (but which we don't want to go to the length of declaring errors, e.g., for backwards-compatibility reasons). These include:

• #[derive()]
• #[must_use] on functions with unit or uninhabited return type
• #[allow()], #[warn()], &c.
• Emit unused_attributes if a level attr only has a reason #94580

[varkor]

I think "#[must_use] on functions with unit or uninhabited return type" should be a Clippy lint rather than a compiler lint.

[Centril]

I've stricken #[must_use] per @varkor's note.

[ehuss]

Posted #88681 which adds some checks for empty attributes like #[allow()].