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/multiModuleHighlighting/multiplatform/nestedClassWithoutImpl/common/common.kt

@@ -0,0 +1,15 @@
+package a
+
+header class A {
+    class Nested
+}
+
+header class B {
+    class Nested {
+        fun foo(s: String)
+    }
+}
+
+header class C {
+    <error>header</error> inner class Inner
+}

idea/testData/multiModuleHighlighting/multiplatform/nestedClassWithoutImpl/jvm/jvm.kt

@@ -0,0 +1,11 @@
+package a
+
+impl class <error>A</error>
+
+impl class B {
+    impl class <error>Nested</error>
+}
+
+impl class C {
+    impl inner class Inner
+}