fix(ivy): align DebugNode/DebugElement behavior with View Engine (#30756)

Two issues with DebugNode/DebugElement in Ivy were causing problems in user
tests.

1. The DebugNodes returned by Ivy were not actually instances of DebugNode.

This was due to an issue with the Ivy switch logic in debug_node.ts.
The declaration of the exported DebugNode reference was set to
`DebugNode__PRE_R3__ as any`. The cast prevented the Ivy switch transform
from detecting this as a switchable declaration. The transform cannot handle
arbitrary syntax, and exports *must* be of the form "const x = y__PRE_R3__;"
or they will not work. The cast to any in this case was not needed, so this
commit removes it.

2. DebugNodes returned by Ivy multiple times for the same element were not
reference-identical. This was previously considered a minor breaking change
in Ivy, but testing has shown that users depend on referential equality of
DebugNodes. This commit caches a DebugNode on a DOM node when first creating
it, to allow returning the same instance in subsequent operations.

PR Close #30756

packages/core/test/debug/debug_node_spec.ts

@@ -7,7 +7,7 @@
  */
 
 
-import {Component, Directive, ElementRef, EmbeddedViewRef, EventEmitter, HostBinding, Injectable, Input, NO_ERRORS_SCHEMA, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
+import {Component, DebugNode, Directive, ElementRef, EmbeddedViewRef, EventEmitter, HostBinding, Injectable, Input, NO_ERRORS_SCHEMA, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
 import {ComponentFixture, TestBed, async} from '@angular/core/testing';
 import {By} from '@angular/platform-browser/src/dom/debug/by';
 import {getDOM} from '@angular/platform-browser/src/dom/dom_adapter';
@@ -678,5 +678,22 @@ class TestCmptWithPropBindings {
       expect(divB.nativeElement.getAttribute('id')).toBe('b');
     });
 
+    it('should be an instance of DebugNode', () => {
+      fixture = TestBed.createComponent(ParentComp);
+      fixture.detectChanges();
+      expect(fixture.debugElement).toBeAnInstanceOf(DebugNode);
+    });
+
+    it('should return the same element when queried twice', () => {
+      fixture = TestBed.createComponent(ParentComp);
+      fixture.detectChanges();
+
+      const childTestElsFirst = fixture.debugElement.queryAll(By.css('child-comp'));
+      const childTestElsSecond = fixture.debugElement.queryAll(By.css('child-comp'));
+
+      expect(childTestElsFirst.length).toBe(1);
+      expect(childTestElsSecond[0]).toBe(childTestElsFirst[0]);
+    });
+
   });
 }