fix(ivy): narrow NgIf context variables in template type checker (#35125)

When the `NgIf` directive is used in a template, its context variables
can be used to capture the bound value. This is typically used together
with a pipe or function call, where the resulting value is captured in a
context variable. There's two syntax forms available:

1. Binding to `NgIfContext.ngIf` using the `as` syntax:
```html
<span *ngIf="(user$ | async) as user">{{user.name}}</span>
```

2. Binding to `NgIfContext.$implicit` using the `let` syntax:
```html
<span *ngIf="user$ | async; let user">{{user.name}}</span>
```

Because of the semantics of `ngIf`, it is known that the captured
context variable is non-nullable, however the template type checker
would not consider them as such and still report errors when
`strictNullTypes` is enabled.

This commit updates `NgIf`'s context guard to make the types of the
context variables non-nullable, avoiding the issue.

Fixes #34572

PR Close #35125

tools/public_api_guard/common/common.d.ts

@@ -236,7 +236,7 @@ export declare class NgIf<T = unknown> {
     set ngIfThen(templateRef: TemplateRef<NgIfContext<T>> | null);
     constructor(_viewContainer: ViewContainerRef, templateRef: TemplateRef<NgIfContext<T>>);
     static ngTemplateGuard_ngIf: 'binding';
-    static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<T>;
+    static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<NonNullable<T>>;
 }
 
 export declare class NgIfContext<T = unknown> {