JVM IR: write inherited multifile parts flag to kotlin.Metadata

This flag is unused at the moment, but might be used one day to support
proper incremental compilation for multifile classes.

compiler/testData/codegen/box/multifileClasses/metadataFlag.kt

@@ -0,0 +1,50 @@
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+// FIR reports unresolved reference on `extraInt`.
+// IGNORE_BACKEND_FIR: JVM_IR
+
+// MODULE: optimized
+// !INHERIT_MULTIFILE_PARTS
+// FILE: optimized.kt
+
+@file:JvmMultifileClass
+@file:JvmName("Optimized")
+package optimized
+
+fun test() {}
+
+class RandomClass
+
+// MODULE: main(optimized)
+// FILE: default.kt
+
+@file:JvmMultifileClass
+@file:JvmName("Default")
+package default
+
+fun test() {}
+
+// FILE: box.kt
+
+fun isFlagSet(className: String): Boolean {
+    val extraInt = Class.forName(className).getAnnotation(Metadata::class.java).extraInt
+    return (extraInt and (1 shl 0)) != 0
+}
+
+fun box(): String {
+    if (isFlagSet("default.Default"))
+        return "Fail: inherited multifile parts flag should NOT be set by default for the facade"
+    if (isFlagSet("default.Default__DefaultKt"))
+        return "Fail: inherited multifile parts flag should NOT be set by default for the part"
+
+    if (!isFlagSet("optimized.Optimized"))
+        return "Fail: inherited multifile parts flag SHOULD be set by default for the facade"
+    if (!isFlagSet("optimized.Optimized__OptimizedKt"))
+        return "Fail: inherited multifile parts flag SHOULD be set by default for the part"
+
+    if (isFlagSet("optimized.RandomClass"))
+        return "Fail: inherited multifile parts flag should NOT be set by default for some random class if -Xmultifile-part-inherit is enabled"
+
+    return "OK"
+}