Java to Kotlin converter: correct nullability in signatures of overrides

#KT-5269 Fixed

j2k/tests/testData/ast/nullability/IndirectOverride.java

@@ -0,0 +1,4 @@
+//file
+class C extends javaApi.Derived {
+    public String foo(String s) { return s; }
+}

j2k/tests/testData/ast/nullability/IndirectOverride.kt

@@ -0,0 +1,5 @@
+class C() : javaApi.Derived() {
+    override fun foo(s: String?): String? {
+        return s
+    }
+}

j2k/tests/testData/ast/nullability/OverrideWithInheritanceLoop.java

@@ -0,0 +1,8 @@
+//file
+class A extends B {
+    public void foo(String s) {}
+}
+
+class B extends A {
+    public void foo(String s) {}
+}

j2k/tests/testData/ast/nullability/OverrideWithInheritanceLoop.kt

@@ -0,0 +1,9 @@
+class A() : B() {
+    public fun foo(s: String) {
+    }
+}
+
+class B() : A() {
+    public fun foo(s: String) {
+    }
+}

j2k/tests/testData/ast/nullability/Overrides.java

@@ -0,0 +1,26 @@
+//file
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+class Base {
+    @Nullable
+    public String foo(@Nullable String s) { return ""; }
+
+    public String bar(String s) {
+        return s != null ? s + 1 : null;
+    }
+
+    public String zoo(Object o){ return ""; }
+}
+
+interface I {
+    @Nullable String zoo(@Nullable Object o);
+}
+
+class C extends Base implements I {
+    public String foo(String s) { return ""; }
+
+    public String bar(String s) { return ""; }
+
+    public String zoo(Object o) { return ""; }
+}

j2k/tests/testData/ast/nullability/Overrides.kt

@@ -0,0 +1,31 @@
+class Base() {
+    public fun foo(s: String?): String? {
+        return ""
+    }
+
+    public fun bar(s: String?): String? {
+        return if (s != null) s + 1 else null
+    }
+
+    public fun zoo(o: Any): String {
+        return ""
+    }
+}
+
+trait I {
+    public fun zoo(o: Any?): String?
+}
+
+class C() : Base(), I {
+    override fun foo(s: String?): String? {
+        return ""
+    }
+
+    override fun bar(s: String?): String? {
+        return ""
+    }
+
+    override fun zoo(o: Any?): String? {
+        return ""
+    }
+}