Kotlin: Record "file class" info from the compiler

We were making file classes when the parent was a file, but not when it
was a class marked as one.

java/kotlin-extractor/src/main/kotlin/KotlinFileExtractor.kt

@@ -643,6 +643,10 @@ open class KotlinFileExtractor(
                         logger.warnElement("Unrecognised class kind $kind", c)
                     }
 
+                    if (c.origin == IrDeclarationOrigin.FILE_CLASS) {
+                        tw.writeFile_class(id)
+                    }
+
                     if (c.isData) {
                         tw.writeKtDataClasses(id)
                     }

java/ql/integration-tests/all-platforms/kotlin/file_classes/classes.expected

@@ -1,3 +1,3 @@
-| AKt.class:0:0:0:0 | AKt | false |
+| AKt.class:0:0:0:0 | AKt | true |
 | B.kt:0:0:0:0 | BKt | true |
 | C.kt:1:1:3:1 | C | false |