feat(compiler): narrow types of expressions used in *ngIf (#20702)

Structural directives can now specify a type guard that describes
what types can be inferred for an input expression inside the
directive's template.

NgIf was modified to declare an input guard on ngIf.

After this change, `fullTemplateTypeCheck` will infer that
usage of `ngIf` expression inside it's template is truthy.

For example, if a component has a property `person?: Person`
and a template of `<div *ngIf="person"> {{person.name}} </div>`
the compiler will no longer report that `person` might be null or
undefined.

The template compiler will generate code similar to,

```
  if (NgIf.ngIfTypeGuard(instance.person)) {
    instance.person.name
  }
```

to validate the template's use of the interpolation expression.
Calling the type guard in this fashion allows TypeScript to infer
that `person` is non-null.

Fixes: #19756?

PR Close #20702

packages/compiler/src/aot/compiler.ts

@@ -271,7 +271,7 @@ export class AotCompiler {
     const {template: parsedTemplate, pipes: usedPipes} =
         this._parseTemplate(compMeta, moduleMeta, directives);
     ctx.statements.push(...this._typeCheckCompiler.compileComponent(
-        componentId, compMeta, parsedTemplate, usedPipes, externalReferenceVars));
+        componentId, compMeta, parsedTemplate, usedPipes, externalReferenceVars, ctx));
   }
 
   emitMessageBundle(analyzeResult: NgAnalyzedModules, locale: string|null): MessageBundle {