JVM: Allow the JvmSynthetic annotation on file classes (KT-41884)

compiler/testData/writeFlags/class/accessFlags/syntheticFile.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+// FILE: test.kt
+@file:Suppress("WRONG_ANNOTATION_TARGET_WITH_USE_SITE_TARGET")
+@file:JvmSynthetic
+
+private val keepme = 1
+
+// TESTED_OBJECT_KIND: class
+// TESTED_OBJECTS: TestKt
+// FLAGS: ACC_FINAL, ACC_PUBLIC, ACC_SUPER, ACC_SYNTHETIC