Function unsupported_dialect_feature_diagnostic 

pub fn unsupported_dialect_feature_diagnostic(
    feature: OracleFeature,
    target: OracleTargetVersion,
    span: Option<Span>,
) -> Option<Diagnostic>

Build a Diagnostic reporting that feature is not available on the parse_options.oracle_version target.

Returns None when the feature is supported on the target — callers can use this as both a check and a builder.