Auto merge of #108402 - clubby789:diag-bool-not-unit, r=davidtwco

Allow using `bool` instead of `Option<()>` in diagnostics

~~Disallow the unit type for `#[help]`, `#[note]` etc, instead using `bool` to express optional annotations without a span which I believe is more intuitive.~~

~~Test output ordering has changed in a few places, where a field was of type `()` and the annotation has been moved to the struct itself. If any of these changes are an issue, this can be restricted to allowing specifically `()`, and not `Option<()>`~~

~~Actual changes here: https://github.com/rust-lang/rust/pull/108402/files#diff-815b1d8debfc564112bd51093791d7c3f2ee288a37a8f5c0e89c11d1f609b4c0~~

Allows using `bool` in derive diagnostics to indicate an optional subdiagnostic without a span, where previously `Option<()>` had to be used

`@rustbot` label +A-diagnostics

Author: bors