fix &str type check in from_str_radix_10

[zihan0822]

minor fix in from_str_radix_10 lint, is_type_diagnostic_item only checks Adt, use .is_str() instead

changelog: none

[rustbot]

r? @samueltardieu

rustbot has assigned @samueltardieu.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[samueltardieu]

Could you add a test where that makes a difference in output? Such as:

    let string = &&&"300";
    i32::from_str_radix(&string, 10)?;

[zihan0822]

Done! @rustbot ready