fix(core): missing-injectable migration should not migrate providers with "useExisting" (#33286)

We should not migrate the reference from `useExisting`. This is because
developers can only use the `useExisting` value as a token. e.g.

```ts
@NgModule({
  providers: [
    {provide: AppRippleConfig, useValue: rippleOptions},
    {provide: MAT_RIPPLE_OPTIONS, useExisting: AppRippleConfig},
  ]
})
export class AppModule {}
```

In the case above, nothing should be decorated with `@Injectable`. The
`AppRippleConfig` class is just used as a token for injection.

PR Close #33286

integration/ng_update_migrations/src/app/migration-tests/providers-test-module.ts

@@ -8,7 +8,7 @@ export class ComponentProvider2 {}
 @Component({
   template: '',
   viewProviders: [ComponentTypeProvider, [
-    {provide: ComponentDontNeedCase, useExisting: ComponentProvider}]
+    {provide: ComponentDontNeedCase, useClass: ComponentProvider}]
   ],
   providers: [ComponentProvider2]
 })
@@ -21,7 +21,7 @@ export class DirectiveProvider {}
 @Directive({
   selector: 'test-dir',
   providers: [DirectiveTypeProvider, [
-    {provide: DirectiveDontNeedCase, useExisting: DirectiveProvider}]
+    {provide: DirectiveDontNeedCase, useClass: DirectiveProvider}]
   ],
 })
 export class ProvidersTestDirective {}

integration/ng_update_migrations/src/app/migration-tests/providers-test-module_expected.ts

@@ -11,7 +11,7 @@ export class ComponentProvider2 {}
 @Component({
   template: '',
   viewProviders: [ComponentTypeProvider, [
-    {provide: ComponentDontNeedCase, useExisting: ComponentProvider}]
+    {provide: ComponentDontNeedCase, useClass: ComponentProvider}]
   ],
   providers: [ComponentProvider2]
 })
@@ -26,7 +26,7 @@ export class DirectiveProvider {}
 @Directive({
   selector: 'test-dir',
   providers: [DirectiveTypeProvider, [
-    {provide: DirectiveDontNeedCase, useExisting: DirectiveProvider}]
+    {provide: DirectiveDontNeedCase, useClass: DirectiveProvider}]
   ],
 })
 export class ProvidersTestDirective {}