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

libraries/stdlib/common/src/kotlin/TextH.kt

@@ -58,7 +58,7 @@ header class Regex {
     fun split(input: CharSequence): List<String>
     fun split(input: CharSequence, limit: Int): List<String>
 
-    header companion object {
+    companion object {
         fun fromLiteral(literal: String): Regex
         fun escape(literal: String): String
         fun escapeReplacement(literal: String): String
@@ -200,4 +200,4 @@ header fun String.toFloatOrNull(): Float?
 
 @PublishedApi
 internal header fun checkRadix(radix: Int): Int
-internal header fun digitOf(char: Char, radix: Int): Int
+internal header fun digitOf(char: Char, radix: Int): Int