Implement stub building for inner types

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

@@ -0,0 +1,17 @@
+// IntelliJ API Decompiler stub source generated from a class file
+// Implementation of methods is not available
+
+package test
+
+public final class InnerClasses<E, F> public constructor() {
+    public final fun bar(x: test.InnerClasses<kotlin.String, kotlin.Double>.Inner2, y: test.InnerClasses<E, F>.Inner2): kotlin.Unit { /* compiled code */ }
+
+    public final inner class Inner<G, H> public constructor() {
+        public final inner class Inner3<I> public constructor() {
+            public final fun foo(x: test.InnerClasses<kotlin.String, F>.Inner<G, kotlin.Int>, y: test.InnerClasses<E, F>.Inner<E, kotlin.Double>, z: test.InnerClasses<kotlin.String, F>.Inner<G, kotlin.Int>.Inner3<kotlin.Double>, w: test.InnerClasses<E, F>.Inner<G, H>.Inner3<*>): kotlin.Unit { /* compiled code */ }
+        }
+    }
+
+    public final inner class Inner2 public constructor() {
+    }
+}

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

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+// 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