Private visibility for non-const, non-jvmField class companion property backing field

jps-plugin/testData/incremental/withJava/kotlinUsedInJava/constantChanged/const.kt

@@ -3,6 +3,6 @@ package test
 class Klass {
     companion object {
         // Old and new constant values are different, but their hashes are the same
-        val CONST = "BF"
+        const val CONST = "BF"
     }
 }

jps-plugin/testData/incremental/withJava/kotlinUsedInJava/constantChanged/const.kt.new

@@ -3,6 +3,6 @@ package test
 class Klass {
     companion object {
         // Old and new constant values are different, but their hashes are the same
-        val CONST = "Ae"
+        const val CONST = "Ae"
     }
 }