Implement --check-cfg option (RFC 3013)

Co-authored-by: Urgau <lolo.branstett@numericable.fr>
Co-authored-by: Marcelina Kościelnicka <mwk@0x04.net>

compiler/rustc_span/src/symbol.rs

@@ -908,6 +908,7 @@ symbols! {
         naked,
         naked_functions,
         name,
+        names,
         native_link_modifiers,
         native_link_modifiers_as_needed,
         native_link_modifiers_bundle,
@@ -1478,6 +1479,7 @@ symbols! {
         va_list,
         va_start,
         val,
+        values,
         var,
         variant_count,
         vec,