IR KT-44233 support flexible nullability in IrTypeSystemContext

^KT-44233 Fixed Target versions 1.5-M1

compiler/testData/codegen/box/collections/kt44233.kt

@@ -0,0 +1,18 @@
+// TARGET_BACKEND: JVM
+// IGNORE_BACKEND_FIR: JVM_IR
+// FULL_JDK
+
+import java.util.concurrent.ConcurrentSkipListSet
+
+class StringIterable : Iterable<String> {
+    private val strings = ConcurrentSkipListSet<String>()
+    override fun iterator() = strings.iterator()
+}
+
+fun box(): String {
+    val si = StringIterable()
+    return if (si.iterator().hasNext())
+        "Failed"
+    else
+        "OK"
+}