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()
+}

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

@@ -0,0 +1,15 @@
+@kotlin.Metadata
+public final class Test1 {
+    // source: 'kt44233.kt'
+    private final field received: java.util.concurrent.ConcurrentSkipListSet
+    public method <init>(): void
+    public @org.jetbrains.annotations.NotNull method iterator(): java.util.Iterator
+}
+
+@kotlin.Metadata
+public final class Test2 {
+    // source: 'kt44233.kt'
+    private final field received: java.lang.String[]
+    public method <init>(): void
+    public @org.jetbrains.annotations.NotNull method iterator(): java.util.Iterator
+}

compiler/testData/codegen/bytecodeListing/collectionStubs/kt44233_ir.txt

@@ -0,0 +1,15 @@
+@kotlin.Metadata
+public final class Test1 {
+    // source: 'kt44233.kt'
+    private final @org.jetbrains.annotations.NotNull field received: java.util.concurrent.ConcurrentSkipListSet
+    public method <init>(): void
+    public @org.jetbrains.annotations.NotNull method iterator(): java.util.Iterator
+}
+
+@kotlin.Metadata
+public final class Test2 {
+    // source: 'kt44233.kt'
+    private final @org.jetbrains.annotations.NotNull field received: java.lang.String[]
+    public method <init>(): void
+    public @org.jetbrains.annotations.NotNull method iterator(): java.util.Iterator
+}