IR KT-44233 support flexible nullability in IrTypeSystemContext

^KT-44233 Fixed Target versions 1.5-M1

compiler/testData/codegen/bytecodeListing/collectionStubs/kt44233.kt

@@ -0,0 +1,14 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// FULL_JDK
+
+import java.util.concurrent.*
+
+class Test1 : Iterable<String> {
+    private val received = ConcurrentSkipListSet<String>()
+    override fun iterator() = received.iterator()
+}
+
+class Test2 : Iterable<String> {
+    private val received = Array<String>(0) { "" }
+    override fun iterator() = received.iterator()
+}