Don't lookup built-in classes by invalid names

#KT-5869 Fixed

compiler/testData/codegen/box/objects/useAnonymousObjectAsIterator.kt

@@ -0,0 +1,18 @@
+// KT-5869
+
+fun <T> Iterator<T>.iterator(): Iterator<T> = this
+
+fun box(): String {
+    val iterator = object : Iterator<Int> {
+        var i = 0
+        override fun next() = i++
+        override fun hasNext() = i < 5
+    }
+
+    var result = ""
+    for (i in iterator) {
+        result += i
+    }
+
+    return if (result == "01234") "OK" else "Fail $result"
+}