Atomicfu-plugin (JVM IR): fix visibility of wrapper classes generated for static atomics.

Fixes https://github.com/Kotlin/kotlinx-atomicfu/issues/272

plugins/atomicfu/atomicfu-compiler/testData/box/TopLevelTest.kt

@@ -153,7 +153,6 @@ class TopLevelArrayTest {
         val a = abcNode.value
         assertTrue(refArr[3].compareAndSet(a3, a))
     }
-
 }
 
 data class ANode<T>(val b: T)
@@ -174,4 +173,4 @@ fun box(): String {
     arrayTest.testBooleanArray()
     arrayTest.testRefArray()
     return "OK"
-}
+}