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/compiler_util/expression_converter.ts

@@ -90,6 +90,14 @@ export class ConvertPropertyBindingResult {
   constructor(public stmts: o.Statement[], public currValExpr: o.Expression) {}
 }
 
+export enum BindingForm {
+  // The general form of binding expression, supports all expressions.
+  General,
+
+  // Try to generate a simple binding (no temporaries or statements)
+  // otherise generate a general binding
+  TrySimple,
+}
 /**
  * Converts the given expression AST into an executable output AST, assuming the expression
  * is used in property binding. The expression has to be preprocessed via
@@ -97,7 +105,8 @@ export class ConvertPropertyBindingResult {
  */
 export function convertPropertyBinding(
     localResolver: LocalResolver | null, implicitReceiver: o.Expression,
-    expressionWithoutBuiltins: cdAst.AST, bindingId: string): ConvertPropertyBindingResult {
+    expressionWithoutBuiltins: cdAst.AST, bindingId: string,
+    form: BindingForm): ConvertPropertyBindingResult {
   if (!localResolver) {
     localResolver = new DefaultLocalResolver();
   }
@@ -110,6 +119,8 @@ export function convertPropertyBinding(
     for (let i = 0; i < visitor.temporaryCount; i++) {
       stmts.push(temporaryDeclaration(bindingId, i));
     }
+  } else if (form == BindingForm.TrySimple) {
+    return new ConvertPropertyBindingResult([], outputExpr);
   }
 
   stmts.push(currValExpr.set(outputExpr).toDeclStmt(null, [o.StmtModifier.Final]));