refactor(ivy): `TypeScriptReflectionHost.isClass` cannot be a type discriminator (#24897)

The `ReflectionHost` interface that is being implemented only expects a
return value of `boolean`.

Moreover, if you want to extend this class to support non-TS code formats,
e.g. ES5, the result of this call returning true does not mean that the `node`
is a `ClassDeclaration`. It could be a `VariableDeclaration`.

PR Close #24897

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

@@ -127,7 +127,7 @@ export class TypeScriptReflectionHost implements ReflectionHost {
     return map;
   }
 
-  isClass(node: ts.Declaration): node is ts.ClassDeclaration {
+  isClass(node: ts.Declaration): boolean {
     // In TypeScript code, classes are ts.ClassDeclarations.
     return ts.isClassDeclaration(node);
   }