Support @CompileTimeConstant for enum fields.

[copybara-service]

Support @CompileTimeConstant for enum fields.

The support for class fields was introduced in unknown commit. This CL merely extends it to enum fields, considering the similarity between them. There is also a feature request b/149290598 filed a few years back.

Specifically, this CL aims to support the following use case (taken from b/149290598):

public class Example {

  public enum ExampleEnum {
    A("a"), B("b");

    @CompileTimeConstant final String s;

    ExampleEnum(@CompileTimeConstant final String s) {
      this.s = s;
    }

    void invokeCtcMethod() {
      ctcMethod(s);
    }

    private void ctcMethod(@CompileTimeConstant String string) {}
  }
}