This is consistent with all other diagnostics I could find containing
features and enables the use of `DiagSymbolList` for generalizing
diagnostics for unstable library features to multiple features.
We are moving toward forbidding `missing_fragment_specifier` either in
edition 2024 or unconditionally. Make a first step toward this by
ensuring crates that rely on the old behavior are reported when used as
dependencies.
Tracking issue: <https://github.com/rust-lang/rust/issues/128143>
