Report errors when iterator() returns a nullable type

compiler/testData/diagnostics/tests/controlStructures/forWithNullableIterator.kt

@@ -0,0 +1,12 @@
+class Coll {
+  fun iterator(): It? = null
+}
+
+class It {
+  fun next() = 1
+  fun hasNext() = false
+}
+
+fun test() {
+  for (x in <!HAS_NEXT_FUNCTION_NONE_APPLICABLE, NEXT_NONE_APPLICABLE!>Coll()<!>) {}
+}