fix(common): narrow NgIf context variables in template type checker (#36627)

When the `NgIf` directive is used in a template, its context variables
can be used to capture the bound value. This is sometimes used in
complex expressions, 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="enabled && user as u">{{u.name}}</span>
```

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

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

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

Based on https://github.com/angular/angular/pull/35125

PR Close #36627

packages/compiler-cli/test/ngtsc/template_typecheck_spec.ts

@@ -72,7 +72,7 @@ export declare class NgIf<T = unknown> {
   ngIfThen: 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<NonNullable<T>>;
+  static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<Exclude<T, false | 0 | "" | null | undefined>>;
   static ɵdir: i0.ɵɵDirectiveDefWithMeta<NgIf<any>, '[ngIf]', never, {'ngIf': 'ngIf'}, {}, never>;
 }
 
@@ -818,7 +818,7 @@ export declare class AnimationEvent {
       template: '<div *ngIf="user; let u">{{u.name}}</div>',
     })
     class TestCmp {
-      user: {name: string}|null;
+      user: {name: string}|null|false;
     }
 
     @NgModule({
@@ -842,7 +842,7 @@ export declare class AnimationEvent {
       template: '<div *ngIf="user as u">{{u.name}}</div>',
     })
     class TestCmp {
-      user: {name: string}|null;
+      user: {name: string}|null|false;
     }
 
     @NgModule({