Don't use private interfaces in stdlib.

This is a murky area which exists just because jvm allows that.

runtime/src/main/kotlin/kotlin/text/Regex.kt

@@ -7,7 +7,8 @@ package kotlin.text
 
 import kotlin.text.regex.*
 
-private interface FlagEnum {
+@PublishedApi
+internal interface FlagEnum {
     val value: Int
     val mask: Int
 }