[BETA_JAVA28] Implement `identity` warnings for value classes (and value bases classes)

https://openjdk.org/jeps/401 reads:

`The garbage collection APIs in the [java.lang.ref](https://download.java.net/java/early_access/jdk28/docs/api/java.base/java/lang/ref/package-summary.html) package and the [java.util.WeakHashMap](https://download.java.net/java/early_access/jdk28/docs/api/java.base/java/util/WeakHashMap.html) class cannot be used with value objects. Attempting to create Reference objects for value objects will cause an IdentityException to be thrown.

Since JDK 25, javac has issued [identity warnings](https://bugs.openjdk.org/browse/JDK-8354556) when value-based classes are used with these APIs. As of JDK 28, javac also issues identity warnings when value classes are used with these APIs.`

ECJ should consider implementing similar warnings.

See that this is not a language compliance issue. A compliant Java compiler is _not_ mandated to issue these warnings. The draft JLS specification is silent about these warnings - see https://download.java.net/java/early_access/jdk28/docs/specs/value-objects-jls.html

Start by reading JEP 401, the draft value-objects JLS specification, and the linked JDK-8354556 issue to understand the identity-warning behavior. Then locate ECJ's diagnostics for java.lang.ref APIs and WeakHashMap; done means ECJ emits comparable warnings for value-based and value classes without treating them as language-compliance errors.

Written by the indexing model from the issue text.