Re-add the "perhaps you meant to use an outer attribute" hint to target checking

[JonathanBrouwer]

#![rustc_main]
mod test {

}

Current output

error: `#[rustc_main]` attribute cannot be used on crates
  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:12:1
   |
LL | #![rustc_main]
   | ^^^^^^^^^^^^^^
   |
   = help: `#[rustc_main]` can only be applied to functions

Desired output

error: `#[rustc_main]` attribute cannot be used on crates
  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:12:1
   |
LL | #![rustc_main]
   | ^^^^^^^^^^^^^^
   |
   = help: `#[rustc_main]` can only be applied to functions. perhaps you meant to use an outer attribute
   |
LL - #![rustc_main]
LL + #[rustc_main]

This hint was there before the target checking refactor but not after, and I think it is a nice hint for people who are a little confused about the attribute syntax of rust and accidentally used !.

[jdonszelmann]

this used to be part of some logic for attrs that are never valid on the root crate. We can easily infer that from the target list so it seems like just a special case for attributes specified on the crate?

[JonathanBrouwer]

We could make this more general, and also give the hint if the attribute is applied to a module (or even other things?) using the inner syntax, while it is not valid on modules

[jdonszelmann]

This seems to be quite complicated, the old check used to find the first item in said module/crate and only suggest when the item is valid on that item. That forces us to defer to when hir is constructed, when possibly spans aren't entirely available

[estebank]

That forces us to defer to when hir is constructed, when possibly spans aren't entirely available

Don't we only need access to the AST node type?