[AA decompiler] Store sealed modifiers in interface stubs

- If the `sealed` modifier is not kept in the interface's stub, a
  sealed interface will have an `abstract` modality instead, which
  causes issues with `when` exhaustiveness analysis in K2.
- This issue also affected K1, but it was less visible there because
  `when` exhaustiveness analysis in K1 uses deserialized descriptors.

^KT-62895

analysis/decompiled/decompiler-to-stubs/src/org/jetbrains/kotlin/analysis/decompiler/stub/ClassClsStubBuilder.kt

@@ -103,6 +103,7 @@ private class ClassClsStubBuilder(
         }
         if (isInterface()) {
             relevantFlags.add(FUN_INTERFACE)
+            relevantFlags.add(INTERFACE_MODALITY)
         }
         val additionalModifiers = when (classKind) {
             ProtoBuf.Class.Kind.ENUM_CLASS -> listOf(KtTokens.ENUM_KEYWORD)

analysis/decompiled/decompiler-to-stubs/src/org/jetbrains/kotlin/analysis/decompiler/stub/flags/flags.kt

@@ -24,6 +24,16 @@ val MODALITY: FlagsToModifiers = object : FlagsToModifiers() {
     }
 }
 
+/**
+ * For interfaces, we only need to remember a `sealed` modifier in the stub, as the interface will be abstract by default.
+ */
+val INTERFACE_MODALITY: FlagsToModifiers = object : FlagsToModifiers() {
+    override fun getModifiers(flags: Int): KtModifierKeywordToken? {
+        val modality = Flags.MODALITY.get(flags)
+        return KtTokens.SEALED_KEYWORD.takeIf { modality == ProtoBuf.Modality.SEALED }
+    }
+}
+
 val VISIBILITY: FlagsToModifiers = object : FlagsToModifiers() {
     override fun getModifiers(flags: Int): KtModifierKeywordToken? {
         val visibility = Flags.VISIBILITY.get(flags)