[FIR] Move check for _private-to-this_ visibility into checker

^KT-55446
^KT-65790 Fixed

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/LookupTagUtils.kt

@@ -61,6 +61,12 @@ fun ConeClassLikeLookupTag.toSymbol(useSiteSession: FirSession): FirClassLikeSym
     }
 }
 
+/**
+ * @see toSymbol
+ */
+fun ConeClassLikeLookupTag.toClassSymbol(session: FirSession): FirClassSymbol<*>? =
+    toSymbol(session) as? FirClassSymbol<*>
+
 /**
  * @see toSymbol
  */