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/nested/header/header.kt

@@ -4,10 +4,10 @@ header class <caret>WithNested {
     fun foo(): Int
 
     class Nested {
-        fun bar() = "Nested"
+        fun bar()
     }
 
     inner class Inner {
-        fun baz() = "Inner"
+        fun baz()
     }
-}
+}

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

@@ -4,10 +4,10 @@ header class WithNested {
     fun foo(): Int
 
     class Nested {
-        fun bar() = "Nested"
+        fun bar()
     }
 
     inner class Inner {
-        fun baz() = "Inner"
+        fun baz()
     }
-}
+}

idea/testData/multiModuleQuickFix/nested/jvm/WithNested.kt.after

@@ -2,11 +2,11 @@
 impl class WithNested {
 
     class Nested {
-        fun bar() = "Nested"
+        fun bar()
     }
 
     inner class Inner {
-        fun baz() = "Inner"
+        fun baz()
     }
 
     impl fun foo(): Int {