Fix isa swizzling for Kotlin subclasses of Obj-C classes

Don't rely on indexed ivars of instance's isa since isa swizzling
replaces it. Use method instead, it works on the replacement too
(if it is a subclass of the original isa).

 #KT-42482 Fixed.

kotlin-native/backend.native/tests/interop/objc/tests/kt42482.h

@@ -0,0 +1,10 @@
+#import <Foundation/NSObject.h>
+
+extern BOOL kt42482Deallocated;
+extern id kt42482Global;
+
+@interface KT42482 : NSObject
+-(int)fortyTwo;
+@end;
+
+void kt42482Swizzle(id obj);

kotlin-native/backend.native/tests/interop/objc/tests/kt42482.kt

@@ -0,0 +1,35 @@
+import kotlin.native.ref.WeakReference
+import kotlinx.cinterop.*
+import kotlin.test.*
+import objcTests.*
+
+@Test
+fun testKT42482() {
+    // Attempt to make the state predictable:
+    kotlin.native.internal.GC.collect()
+
+    kt42482Deallocated = false
+    assertFalse(kt42482Deallocated);
+
+    {
+        assertEquals(41, KT42482().fortyTwo())
+
+        val obj: KT42482 = KT42482Impl()
+        assertEquals(42, obj.fortyTwo())
+
+        kt42482Swizzle(obj)
+        assertEquals(43, obj.fortyTwo())
+
+        // Test retain and release on swizzled object:
+        kt42482Global = obj
+        kt42482Global = null
+    }()
+
+    kotlin.native.internal.GC.collect()
+
+    assertTrue(kt42482Deallocated)
+}
+
+class KT42482Impl : KT42482() {
+    override fun fortyTwo() = 42
+}

kotlin-native/backend.native/tests/interop/objc/tests/kt42482.m

@@ -0,0 +1,36 @@
+#import "kt42482.h"
+#import "assert.h"
+#import <objc/runtime.h>
+
+BOOL kt42482Deallocated = NO;
+id kt42482Global = nil;
+
+@implementation KT42482
+-(int)fortyTwo {
+    return 41;
+}
+
+-(void)dealloc {
+    kt42482Deallocated = YES;
+}
+@end;
+
+int fortyTwoSwizzledImp(id self, SEL _cmd) {
+    return 43;
+}
+
+void kt42482Swizzle(id obj) {
+    Class oldClass = object_getClass(obj);
+
+    SEL selector = @selector(fortyTwo);
+
+    Class newClass = objc_allocateClassPair(oldClass, "KT42482Swizzled", 0);
+    assert(newClass != nil);
+    objc_registerClassPair(newClass);
+
+    Method method = class_getInstanceMethod([KT42482 class], selector);
+    assert(method != nil);
+    class_addMethod(newClass, selector, (IMP)&fortyTwoSwizzledImp, method_getTypeEncoding(method));
+
+    object_setClass(obj, newClass);
+}