fix(compiler): fix a few non-tree-shakeable code patterns (#24677)

This change makes @angular/compiler more tree-shakeable by changing
an enum to a const enum and by getting rid of a top-level map that
the tree-shaker was seeing as a reference which caused r3_identifiers
to be retained.

This drops a few hundred bytes of JS from tree-shaken ngtsc compiled
apps.

PR Close #24677

packages/compiler/src/template_parser/template_ast.ts

@@ -58,7 +58,7 @@ export class AttrAst implements TemplateAst {
   visit(visitor: TemplateAstVisitor, context: any): any { return visitor.visitAttr(this, context); }
 }
 
-export enum PropertyBindingType {
+export const enum PropertyBindingType {
   // A normal binding to a property (e.g. `[property]="expression"`).
   Property,
   // A binding to an element attribute (e.g. `[attr.name]="expression"`).