KT-6530 Do not report "useless cast" when RHS is unresolved

#KT-6530 fixed

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/BasicExpressionTypingVisitor.java

@@ -205,7 +205,7 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
             JetType targetType,
             ExpressionTypingContext context
     ) {
-        if (actualType == null || noExpectedType(targetType)) return;
+        if (actualType == null || noExpectedType(targetType) || targetType.isError()) return;
 
         if (TypesPackage.isDynamic(targetType)) {
             JetTypeReference right = expression.getRight();

compiler/testData/diagnostics/tests/Casts.kt

@@ -13,4 +13,6 @@ fun test() : Unit {
   x <!USELESS_CAST!>as? Int?<!> : Int?
   y <!USELESS_CAST_STATIC_ASSERT_IS_FINE!>as? Int?<!> : Int?
   Unit
+
+  x as <!UNRESOLVED_REFERENCE!>UnresolvedType<!>
 }