Don't emit CAST_NEVER_SUCCEEDS when casting to a forward declaration type

^KT-58929

core/descriptors/src/org/jetbrains/kotlin/builtins/PlatformSpecificCastChecker.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+package org.jetbrains.kotlin.builtins
+
+import org.jetbrains.kotlin.container.DefaultImplementation
+import org.jetbrains.kotlin.types.KotlinType
+
+@DefaultImplementation(impl = PlatformSpecificCastChecker.Default::class)
+interface PlatformSpecificCastChecker {
+    fun isCastPossible(fromType: KotlinType, toType: KotlinType): Boolean
+
+    class Default : PlatformSpecificCastChecker {
+        override fun isCastPossible(fromType: KotlinType, toType: KotlinType): Boolean {
+            return false
+        }
+    }
+}