Function feature_err_unstable_feature_bound

pub fn feature_err_unstable_feature_bound(
    sess: &Session,
    feature: Symbol,
    span: impl Into<MultiSpan>,
    explain: impl Into<DiagMessage>,
) -> Diag<'_>

This is only used by unstable_feature_bound as it does not have issue number information for now. This is basically the same as feature_err_issue but without the feature issue note. If we can do a lookup for issue number from feature name, then we should directly use feature_err_issue for ambiguity error of #[unstable_feature_bound].