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

compiler/testData/multiplatform/implTypeAlias/nestedClassesViaTypeAlias/common.kt

@@ -1,3 +1,3 @@
 header class ByTypeAlias {
-    header interface Nested
+    interface Nested
 }

compiler/testData/multiplatform/implTypeAlias/nestedClassesViaTypeAlias/output.txt

@@ -3,8 +3,5 @@ Exit code: OK
 Output:
 
 -- JVM --
-Exit code: COMPILATION_ERROR
+Exit code: OK
 Output:
-compiler/testData/multiplatform/implTypeAlias/nestedClassesViaTypeAlias/common.kt:2:22: error: 'header' interface 'Nested' has no implementation in module
-    header interface Nested
-                     ^

compiler/testData/multiplatform/incompatibleNestedClasses/common.kt

@@ -1,19 +1,19 @@
 header class O1 {
-    header class N1
-    header interface N2
-    header object N3
+    class N1
+    interface N2
+    object N3
 }
 
 header class O2 {
-    header class N2
-    header inner class I2
+    class N2
+    inner class I2
 }
 
 header class O3 {
-    header object Companion
-    header companion object Factory
+    object Companion
+    companion object Factory
 }
 
 header class O4 {
-    header companion object
+    companion object
 }