Improved error message for 'nested class should be qualified'

idea/testData/diagnosticMessage/nestedClassShouldBeQualified.kt

@@ -0,0 +1,38 @@
+// !DIAGNOSTICS_NUMBER: 5
+// !DIAGNOSTICS: NESTED_CLASS_SHOULD_BE_QUALIFIED
+
+package p
+
+class A {
+    class B {
+        class Nested
+    }
+}
+
+fun A.B.test() {
+    Nested()
+    ::Nested
+}
+
+class C {
+    class object {
+        class D {
+            class Nested
+        }
+    }
+}
+
+fun C.D.text() {
+    Nested()
+    ::Nested
+}
+
+class E {
+    class F {
+        class object
+    }
+}
+
+fun E.test() {
+    F
+}