EnhancedNullability annotation in IR

Fixes KT-40115 & KT-40117.

Move FlexibleNullability annotation to 'kotlin.internal.ir'.

compiler/testData/ir/irText/classes/delegatedImplementationOfJavaInterface.kt

@@ -0,0 +1,17 @@
+// FILE: delegatedImplementationOfJavaInterface.kt
+
+class Test(private val j: J) : J by j
+
+// FILE: J.java
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public interface J {
+    void takeNotNull(@NotNull String x);
+    void takeNullable(@Nullable String x);
+    void takeFlexible(String x);
+    @NotNull String returnNotNull();
+    @Nullable String returnNullable();
+    String returnsFlexible();
+}