Treat nested class of header class as header

Note that the quick fix to implement header class works incorrectly when
that class has nested classes at the moment; this should be fixed
separately

 #KT-15494 Fixed
 #KT-18573 Fixed

idea/testData/multiModuleQuickFix/enum/header/header.kt

@@ -6,12 +6,4 @@ header enum class <caret>MyEnum {
     LAST;
 
     val num: Int
-
-    companion object {
-        fun byNum(num: Int): MyEnum = when (num) {
-            1 -> FIRST
-            2 -> SECOND
-            else -> LAST
-        }
-    }
-}
+}

idea/testData/multiModuleQuickFix/enum/header/header.kt.after

@@ -6,12 +6,4 @@ header enum class MyEnum {
     LAST;
 
     val num: Int
-
-    companion object {
-        fun byNum(num: Int): MyEnum = when (num) {
-            1 -> FIRST
-            2 -> SECOND
-            else -> LAST
-        }
-    }
-}
+}