Implement stub building for inner types

idea/testData/decompiler/decompiledText/InnerClasses/InnerClasses.kt

@@ -0,0 +1,17 @@
+package test
+
+class InnerClasses<E, F> {
+    inner class Inner<G, H> {
+        inner class Inner3<I> {
+            fun foo(
+                    x: InnerClasses<String, F>.Inner<G, Int>,
+                    y: Inner<E, Double>,
+                    z: InnerClasses<String, F>.Inner<G, Int>.Inner3<Double>,
+                    w: Inner3<*>) {}
+        }
+    }
+
+    inner class Inner2
+
+    fun bar(x: InnerClasses<String, Double>.Inner2, y: Inner2) {}
+}

idea/testData/decompiler/decompiledText/InnerClasses/directives.txt

@@ -0,0 +1,4 @@
+// TODO Remove this restriction when nested classes will be supported in js backend.
+// See KT-1907 Support nested classes in js-backend
+// https://youtrack.jetbrains.com/issue/KT-1907
+// TARGET_BACKEND: JVM