Allopen: Handle allopen annotations on classes (KT-12149)

plugins/allopen/testData/bytecodeListing/alreadyOpen.kt

@@ -0,0 +1,16 @@
+annotation class AllOpen
+
+@AllOpen
+open class Test1
+
+@AllOpen
+open class Test2 {
+    open fun method() {}
+    val prop: String = ""
+}
+
+@AllOpen
+class Test3 {
+    fun method() {}
+    open val prop: String = ""
+}