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/compile_reflector.ts

@@ -17,6 +17,7 @@ export abstract class CompileReflector {
   abstract annotations(typeOrFunc: /*Type*/ any): any[];
   abstract propMetadata(typeOrFunc: /*Type*/ any): {[key: string]: any[]};
   abstract hasLifecycleHook(type: any, lcProperty: string): boolean;
+  abstract guards(typeOrFunc: /* Type */ any): {[key: string]: any};
   abstract componentModuleUrl(type: /*Type*/ any, cmpMetadata: Component): string;
   abstract resolveExternalReference(ref: o.ExternalReference): any;
 }