Add specific ObjCMethodSpec kind for object instance getters

This helps to drop usages of obsolete "IrDeclaration.descriptor"
from ObjCExportCodeGenerator

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/objcexport/ObjCExportCodeGenerator.kt

@@ -1257,6 +1257,13 @@ private fun ObjCExportCodeGenerator.createTypeAdapter(
             is ObjCClassMethodForKotlinEnumValues -> {
                 classAdapters += createEnumValuesAdapter(it.valuesFunctionSymbol.owner, it.selector)
             }
+            is ObjCGetterForObjectInstance -> {
+                classAdapters += if (irClass.isUnit()) {
+                    createUnitInstanceAdapter(it.selector)
+                } else {
+                    createObjectInstanceAdapter(irClass, it.selector)
+                }
+            }
             is ObjCMethodForKotlinMethod -> {} // Handled below.
         }.let {} // Force exhaustive.
     }
@@ -1309,19 +1316,6 @@ private fun ObjCExportCodeGenerator.createTypeAdapter(
         else -> Pair(emptyList(), -1)
     }
 
-    when (irClass.kind) {
-        ClassKind.OBJECT -> {
-            classAdapters += if (irClass.isUnit()) {
-                createUnitInstanceAdapter()
-            } else {
-                createObjectInstanceAdapter(irClass)
-            }
-        }
-        else -> {
-            // Nothing special.
-        }
-    }
-
     return ObjCTypeAdapter(
             irClass,
             typeInfo,
@@ -1464,23 +1458,20 @@ private fun ObjCExportCodeGenerator.objCToKotlinMethodAdapter(
     return ObjCToKotlinMethodAdapter(selector, getEncoding(methodBridge), constPointer(imp))
 }
 
-private fun ObjCExportCodeGenerator.createUnitInstanceAdapter() =
-        generateObjCToKotlinSyntheticGetter(
-                namer.getObjectInstanceSelector(context.builtIns.unit)
-        ) {
+private fun ObjCExportCodeGenerator.createUnitInstanceAdapter(selector: String) =
+        generateObjCToKotlinSyntheticGetter(selector) {
             initRuntimeIfNeeded() // For instance methods it gets called when allocating.
 
             ret(callFromBridge(context.llvm.Kotlin_ObjCExport_convertUnit, listOf(codegen.theUnitInstanceRef.llvm)))
         }
 
 private fun ObjCExportCodeGenerator.createObjectInstanceAdapter(
-        irClass: IrClass
+        irClass: IrClass,
+        selector: String
 ): ObjCExportCodeGenerator.ObjCToKotlinMethodAdapter {
     assert(irClass.kind == ClassKind.OBJECT)
     assert(!irClass.isUnit())
 
-    val selector = namer.getObjectInstanceSelector(irClass.descriptor)
-
     return generateObjCToKotlinSyntheticGetter(selector) {
         initRuntimeIfNeeded() // For instance methods it gets called when allocating.
         val value = getObjectValue(irClass, startLocationInfo = null, exceptionHandler = ExceptionHandler.Caller)

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/objcexport/ObjCExportCodeSpec.kt

@@ -63,6 +63,10 @@ internal fun ObjCExportedInterface.createCodeSpec(symbolTable: SymbolTable): Obj
                 }
             }
 
+            if (descriptor.kind == ClassKind.OBJECT) {
+                methods += ObjCGetterForObjectInstance(namer.getObjectInstanceSelector(descriptor))
+            }
+
             if (descriptor.kind == ClassKind.ENUM_CLASS) {
                 descriptor.enumEntries.mapTo(methods) {
                     ObjCGetterForKotlinEnumEntry(symbolTable.referenceEnumEntry(it))
@@ -112,6 +116,8 @@ internal class ObjCClassMethodForKotlinEnumValues(
         val selector: String
 ) : ObjCMethodSpec()
 
+internal class ObjCGetterForObjectInstance(val selector: String) : ObjCMethodSpec()
+
 internal sealed class ObjCTypeSpec(val binaryName: String)
 
 internal sealed class ObjCTypeForKotlinType(