[Interop][StubIR] Add "abstract" member stub modality

Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/ObjCStubs.kt

@@ -132,7 +132,7 @@ private class ObjCMethodStubBuilder(
                     MemberStubModality.OVERRIDE
                 } else when (container) {
                     is ObjCClass -> MemberStubModality.OPEN
-                    is ObjCProtocol -> MemberStubModality.OPEN
+                    is ObjCProtocol -> MemberStubModality.ABSTRACT
                 }
             }
             is ObjCCategory -> MemberStubModality.FINAL

Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/StubIr.kt

@@ -322,7 +322,8 @@ class FunctionParameterStub(
 enum class MemberStubModality {
     OVERRIDE,
     OPEN,
-    FINAL
+    FINAL,
+    ABSTRACT
 }
 
 interface FunctionalStub : AnnotationHolder, StubIrElement, NativeBacked {

Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/StubIrExtensions.kt

@@ -55,7 +55,7 @@ val StubContainer.defaultMemberModality: MemberStubModality
     get() = when (this) {
         is SimpleStubContainer -> MemberStubModality.FINAL
         is ClassStub.Simple -> if (this.modality == ClassStubModality.INTERFACE) {
-            MemberStubModality.OPEN
+            MemberStubModality.ABSTRACT
         } else {
             MemberStubModality.FINAL
         }

Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/StubIrMetadataEmitter.kt

@@ -116,7 +116,8 @@ private val FunctionStub.flags: Flags
             Flag.Function.IS_EXTERNAL,
             Flag.HAS_ANNOTATIONS.takeIf { annotations.isNotEmpty() },
             Flag.IS_FINAL.takeIf { modality == MemberStubModality.FINAL },
-            Flag.IS_OPEN.takeIf { modality == MemberStubModality.OPEN }
+            Flag.IS_OPEN.takeIf { modality == MemberStubModality.OPEN },
+            Flag.IS_ABSTRACT.takeIf { modality == MemberStubModality.ABSTRACT }
     ).let { flagsOf(*it.toTypedArray()) }
 
 private val Classifier.fqNameSerialized: String

Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/StubIrTextEmitter.kt

@@ -361,6 +361,7 @@ class StubIrTextEmitter(
                     MemberStubModality.OVERRIDE -> "override "
                     MemberStubModality.OPEN -> "open "
                     MemberStubModality.FINAL -> "final "
+                    MemberStubModality.ABSTRACT -> "abstract "
                 }
 
     private fun renderVisibilityModifier(visibilityModifier: VisibilityModifier) = when (visibilityModifier) {