[KT-4124] Add test case for nested/inner classes inside native class. Add diagnostic of inner classes inside native classes.

compiler/testData/diagnostics/testsWithJsStdLib/native/nested/nativeNestedClassProhibited.kt

@@ -0,0 +1,5 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+class A {
+    <!NATIVE_INNER_CLASS_PROHIBITED!>@native inner class B<!>
+}

compiler/testData/diagnostics/testsWithJsStdLib/native/nested/nativeNestedClassProhibited.txt

@@ -0,0 +1,15 @@
+package
+
+public final class A {
+    public constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    @kotlin.js.native() public final inner class B {
+        public constructor B()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}