feat(ivy): capture the identifier of a decorator during reflection (#26860)

Previously the ReflectionHost API only returned the names of decorators
and not a reference to their TypeScript Identifier. This commit adds
the identifier itself, so that a consumer can write references to the
decorator.

Testing strategy: this commit is trivial, and the functionality will be
exercised by downstream tests.

PR Close #26860

packages/compiler-cli/src/ngtsc/host/src/reflection.ts

@@ -20,6 +20,11 @@ export interface Decorator {
    */
   name: string;
 
+  /**
+   * Identifier which refers to the decorator in source.
+   */
+  identifier: ts.Identifier;
+
   /**
    * `Import` by which the decorator was brought into the module in which it was invoked, or `null`
    * if the decorator was declared in the same module and not imported.

packages/compiler-cli/src/ngtsc/metadata/src/reflector.ts

@@ -245,6 +245,7 @@ export class TypeScriptReflectionHost implements ReflectionHost {
 
     return {
       name: decoratorExpr.text,
+      identifier: decoratorExpr,
       import: importDecl, node, args,
     };
   }