Improve diagnostic of the unexpected_cfgs lint

compiler/rustc_lint_defs/src/lib.rs

@@ -310,6 +310,7 @@ pub enum BuiltinLintDiagnostics {
     BreakWithLabelAndLoop(Span),
     NamedAsmLabel(String),
     UnicodeTextFlow(Span, String),
+    UnexpectedCfg(Span, Symbol, Option<Symbol>),
 }
 
 /// Lints that are buffered up early on in the `Session` before the