Introduction of CastedClassReceiver and its handling in codegen #KT-6744 Fixed

ClassReceiver converted to Kotlin
Also #KT-7617 Fixed

compiler/testData/codegen/box/smartCasts/implicitExtensionReceiver.kt

@@ -0,0 +1,7 @@
+class A {
+    fun foo() = "OK"
+}
+
+fun A?.bar() = if (this != null) foo() else "FAIL"
+
+fun box() = A().bar()