EnhancedNullability annotation in IR

Fixes KT-40115 & KT-40117.

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

compiler/testData/codegen/bytecodeListing/nullabilityAnnotations/nullabilityAnnotationsForReturnType.kt

@@ -0,0 +1,17 @@
+// FILE: nullabilityAnnotationsForReturnType.kt
+
+fun testNotNull(): String = ""
+fun testNullable(): String? = ""
+fun testJavaNotNull() = J.returnsNotNull()
+fun testJavaNullable() = J.returnsNullable()
+fun testJavaFlexible() = J.returnsFlexible()
+
+// FILE: J.java
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public class J {
+    public static @NotNull String returnsNotNull() { return ""; }
+    public static @Nullable String returnsNullable() { return ""; }
+    public static String returnsFlexible() { return ""; }
+}

compiler/testData/codegen/bytecodeListing/nullabilityAnnotations/nullabilityAnnotationsForReturnType.txt

@@ -0,0 +1,9 @@
+@kotlin.Metadata
+public final class NullabilityAnnotationsForReturnTypeKt {
+    // source: 'nullabilityAnnotationsForReturnType.kt'
+    public final static method testJavaFlexible(): java.lang.String
+    public final static @org.jetbrains.annotations.NotNull method testJavaNotNull(): java.lang.String
+    public final static @org.jetbrains.annotations.Nullable method testJavaNullable(): java.lang.String
+    public final static @org.jetbrains.annotations.NotNull method testNotNull(): java.lang.String
+    public final static @org.jetbrains.annotations.Nullable method testNullable(): java.lang.String
+}

compiler/testData/codegen/bytecodeListing/nullabilityAnnotations/nullabilityAnnotationsOnDelegatedMembers.kt

@@ -0,0 +1,15 @@
+// FILE: nullabilityAnnotationsOnDelegatedMembers.kt
+class JImpl(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();
+}

compiler/testData/codegen/bytecodeListing/nullabilityAnnotations/nullabilityAnnotationsOnDelegatedMembers.txt

@@ -0,0 +1,12 @@
+@kotlin.Metadata
+public final class JImpl {
+    // source: 'nullabilityAnnotationsOnDelegatedMembers.kt'
+    private synthetic final field $$delegate_0: J
+    public method <init>(@org.jetbrains.annotations.NotNull p0: J): void
+    public @org.jetbrains.annotations.NotNull method returnNotNull(): java.lang.String
+    public @org.jetbrains.annotations.Nullable method returnNullable(): java.lang.String
+    public method returnsFlexible(): java.lang.String
+    public method takeFlexible(p0: java.lang.String): void
+    public method takeNotNull(@org.jetbrains.annotations.NotNull p0: java.lang.String): void
+    public method takeNullable(@org.jetbrains.annotations.Nullable p0: java.lang.String): void
+}