Disallow nested classes within inner & local classes

#KT-1174 In Progress

compiler/testData/diagnostics/tests/inner/nestedClassNotAllowed.kt

@@ -0,0 +1,25 @@
+class A {
+    inner class B {
+        class <!NESTED_CLASS_NOT_ALLOWED!>C<!>
+    }
+    
+    fun foo() {
+        class B {
+            class <!NESTED_CLASS_NOT_ALLOWED!>C<!>
+        }
+    }
+}
+
+fun foo() {
+    class B {
+        class <!NESTED_CLASS_NOT_ALLOWED!>C<!>
+    }
+}
+
+
+enum class E {
+    E1 {
+        // Not allowed in Java, but no reason to disallow in Kotlin
+        class D
+    }
+}